perf tools: Replace zero-length array with flexible-array

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array
member[1][2], introduced in C99:

struct foo {
int stuff;
struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning in
case the flexible array does not occur last in the structure, which will
help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by this
change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Gustavo A. R. Silva <[e-mail]>
Cc: Ian Rogers <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: http://lore.kernel.org/lkml/20200515172926.GA31976@embeddedor
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/bench/sched-messaging.c

···
 	unsigned int num_fds;
 	int ready_out;
 	int wakefd;
-	int out_fds[0];
+	int out_fds[];
 };
 
 struct receiver_context {

tools/perf/builtin-inject.c

···
 struct event_entry {
 	struct list_head node;
 	u32		 tid;
-	union perf_event event[0];
+	union perf_event event[];
 };
 
 static int output_bytes(struct perf_inject *inject, void *buf, size_t sz)

tools/perf/builtin-script.c

···
 struct script_spec {
 	struct list_head	node;
 	struct scripting_ops	*ops;
-	char			spec[0];
+	char			spec[];
 };
 
 static LIST_HEAD(script_specs);

tools/perf/builtin-timechart.c

···
 	struct sample_wrapper *next;
 
 	u64		timestamp;
-	unsigned char	data[0];
+	unsigned char	data[];
 };
 
 #define TYPE_NONE	0

tools/perf/util/annotate.h

···
 	u32			 idx;
 	int			 idx_asm;
 	int			 data_nr;
-	struct annotation_data	 data[0];
+	struct annotation_data	 data[];
 };
 
 struct disasm_line {
···
 struct sym_hist {
 	u64		      nr_samples;
 	u64		      period;
-	struct sym_hist_entry addr[0];
+	struct sym_hist_entry addr[];
 };
 
 struct cyc_hist {

tools/perf/util/cputopo.h

···
 
 struct numa_topology {
 	u32				nr;
-	struct numa_topology_node	nodes[0];
+	struct numa_topology_node	nodes[];
 };
 
 struct cpu_topology *cpu_topology__new(void);

tools/perf/util/dso.h

···
 	struct rb_node	rb_node;
 	u64 offset;
 	u64 size;
-	char data[0];
+	char data[];
 };
 
 struct auxtrace_cache;
···
 	struct nsinfo	*nsinfo;
 	struct dso_id	 id;
 	refcount_t	 refcnt;
-	char		 name[0];
+	char		 name[];
 };
 
 /* dso__for_each_symbol - iterate over the symbols of given type

tools/perf/util/event.h

···
 
 struct ip_callchain {
 	u64 nr;
-	u64 ips[0];
+	u64 ips[];
 };
 
 struct branch_stack;

tools/perf/util/jitdump.c

···
 	unsigned long vma;
 	unsigned int lineno;
 	/* The filename format is unspecified, absolute path, relative etc. */
-	char const filename[0];
+	char const filename[];
 };
 
 struct jit_tool {

tools/perf/util/jitdump.h

···
 	uint64_t addr;
 	int lineno;	    /* source line number starting at 1 */
 	int discrim;	    /* column discriminator, 0 is default */
-	const char name[0]; /* null terminated filename, \xff\0 if same as previous entry */
+	const char name[]; /* null terminated filename, \xff\0 if same as previous entry */
 };
 
 struct jr_code_debug_info {
···
 
 	uint64_t code_addr;
 	uint64_t nr_entry;
-	struct debug_entry entries[0];
+	struct debug_entry entries[];
 };
 
 struct jr_code_unwinding_info {
···
 	uint64_t unwinding_size;
 	uint64_t eh_frame_hdr_size;
 	uint64_t mapped_size;
-	const char unwinding_data[0];
+	const char unwinding_data[];
 };
 
 union jr_entry {

tools/perf/util/ordered-events.h

···
 
 struct ordered_events_buffer {
 	struct list_head	list;
-	struct ordered_event	event[0];
+	struct ordered_event	event[];
 };
 
 struct ordered_events {

tools/perf/util/pstack.c

···
 struct pstack {
 	unsigned short	top;
 	unsigned short	max_nr_entries;
-	void		*entries[0];
+	void		*entries[];
 };
 
 struct pstack *pstack__new(unsigned short max_nr_entries)

tools/perf/util/symbol.h

···
 	u8		inlined:1;
 	u8		arch_sym;
 	bool		annotate2;
-	char		name[0];
+	char		name[];
 };
 
 void symbol__delete(struct symbol *sym);

tools/perf/util/unwind-libunwind-local.c

···
 	 *    encoded_t fde_addr;
 	 * } binary_search_table[fde_count];
 	 */
-	char data[0];
+	char data[];
 } __packed;
 
 static int unwind_spec_ehframe(struct dso *dso, struct machine *machine,