perf tools: Fix arm64 build error with gcc-11

gcc version: 11.0.0 20210208 (experimental) (GCC)

Following build error on arm64:

.......
In function ‘printf’,
inlined from ‘regs_dump__printf’ at util/session.c:1141:3,
inlined from ‘regs__printf’ at util/session.c:1169:2:
/usr/include/aarch64-linux-gnu/bits/stdio2.h:107:10: \
error: ‘%-5s’ directive argument is null [-Werror=format-overflow=]

107 | return __printf_chk (__USE_FORTIFY_LEVEL - 1, __fmt, \
__va_arg_pack ());

......
In function ‘fprintf’,
inlined from ‘perf_sample__fprintf_regs.isra’ at \
builtin-script.c:622:14:
/usr/include/aarch64-linux-gnu/bits/stdio2.h:100:10: \
error: ‘%5s’ directive argument is null [-Werror=format-overflow=]
100 | return __fprintf_chk (__stream, __USE_FORTIFY_LEVEL - 1, __fmt,
101 | __va_arg_pack ());

cc1: all warnings being treated as errors
.......

This patch fixes Wformat-overflow warnings. Add helper function to
convert NULL to "unknown".

Signed-off-by: Jianlin Lv <[e-mail]>
Reviewed-by: John Garry <[e-mail]>
Acked-by: Jiri Olsa <[e-mail]>
Cc: Albert Ou <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Anju T Sudhakar <[e-mail]>
Cc: Athira Jajeev <[e-mail]>
Cc: Guo Ren <[e-mail]>
Cc: Kajol Jain <[e-mail]>
Cc: Leo Yan <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Mathieu Poirier <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Paul Walmsley <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Ravi Bangoria <[e-mail]>
Cc: Will Deacon <[e-mail]>
Cc: Palmer Dabbelt <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Link: http://lore.kernel.org/lkml/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/arch/arm/include/perf_regs.h

···
 #define PERF_REG_IP	PERF_REG_ARM_PC
 #define PERF_REG_SP	PERF_REG_ARM_SP
 
-static inline const char *perf_reg_name(int id)
+static inline const char *__perf_reg_name(int id)
 {
 	switch (id) {
 	case PERF_REG_ARM_R0:

tools/perf/arch/arm64/include/perf_regs.h

···
 #define PERF_REG_IP	PERF_REG_ARM64_PC
 #define PERF_REG_SP	PERF_REG_ARM64_SP
 
-static inline const char *perf_reg_name(int id)
+static inline const char *__perf_reg_name(int id)
 {
 	switch (id) {
 	case PERF_REG_ARM64_X0:

tools/perf/arch/csky/include/perf_regs.h

···
 #define PERF_REG_IP	PERF_REG_CSKY_PC
 #define PERF_REG_SP	PERF_REG_CSKY_SP
 
-static inline const char *perf_reg_name(int id)
+static inline const char *__perf_reg_name(int id)
 {
 	switch (id) {
 	case PERF_REG_CSKY_A0:

tools/perf/arch/powerpc/include/perf_regs.h

···
 	[PERF_REG_POWERPC_PMC6] = "pmc6",
 };
 
-static inline const char *perf_reg_name(int id)
+static inline const char *__perf_reg_name(int id)
 {
 	return reg_names[id];
 }

tools/perf/arch/riscv/include/perf_regs.h

···
 #define PERF_REG_IP	PERF_REG_RISCV_PC
 #define PERF_REG_SP	PERF_REG_RISCV_SP
 
-static inline const char *perf_reg_name(int id)
+static inline const char *__perf_reg_name(int id)
 {
 	switch (id) {
 	case PERF_REG_RISCV_PC:

tools/perf/arch/s390/include/perf_regs.h

···
 #define PERF_REG_IP PERF_REG_S390_PC
 #define PERF_REG_SP PERF_REG_S390_R15
 
-static inline const char *perf_reg_name(int id)
+static inline const char *__perf_reg_name(int id)
 {
 	switch (id) {
 	case PERF_REG_S390_R0:

tools/perf/arch/x86/include/perf_regs.h

···
 #define PERF_REG_IP PERF_REG_X86_IP
 #define PERF_REG_SP PERF_REG_X86_SP
 
-static inline const char *perf_reg_name(int id)
+static inline const char *__perf_reg_name(int id)
 {
 	switch (id) {
 	case PERF_REG_X86_AX:

tools/perf/util/perf_regs.h

···
 
 int perf_reg_value(u64 *valp, struct regs_dump *regs, int id);
 
+static inline const char *perf_reg_name(int id)
+{
+	const char *reg_name = __perf_reg_name(id);
+
+	return reg_name ?: "unknown";
+}
+
 #else
 #define PERF_REGS_MASK	0
 #define PERF_REGS_MAX	0