tools build: Fix a number of Wconversion warnings

There's some expressed interest in having the compiler flag
-Wconversion detect at build time certain kinds of potential problems:
https://lore.kernel.org/lkml/[e-mail]/

As feature detection passes -Wconversion from CFLAGS when set, the
feature detection compile tests need to not fail because of
-Wconversion as the failure will be interpretted as a missing
feature. Switch various types to avoid the -Wconversion issue, the
exact meaning of the code is unimportant as it is typically looking
for header file definitions.

Signed-off-by: Ian Rogers <[e-mail]>
Reviewed-by: James Clark <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Namhyung Kim <[e-mail]>

tools/build/feature/test-backtrace.c

···
 int main(void)
 {
 	void *backtrace_fns[10];
-	size_t entries;
+	int entries;
 
 	entries = backtrace(backtrace_fns, 10);
 	backtrace_symbols_fd(backtrace_fns, entries, 1);

tools/build/feature/test-bpf.c

···
 	 * Test existence of __NR_bpf and BPF_PROG_LOAD.
 	 * This call should fail if we run the testcase.
 	 */
-	return syscall(__NR_bpf, BPF_PROG_LOAD, &attr, sizeof(attr));
+	return syscall(__NR_bpf, BPF_PROG_LOAD, &attr, sizeof(attr)) == 0;
 }

tools/build/feature/test-glibc.c

···
 	const char *version = XSTR(__GLIBC__) "." XSTR(__GLIBC_MINOR__);
 #endif
 
-	return (long)version;
+	return version == NULL;
 }

tools/build/feature/test-libdebuginfod.c

···
 int main(void)
 {
 	debuginfod_client* c = debuginfod_begin();
-	return (long)c;
+	return !!c;
 }

tools/build/feature/test-libdw.c

···
 {
 	Dwarf *dbg = dwarf_begin(0, DWARF_C_READ);
 
-	return (long)dbg;
+	return dbg == NULL;
 }
 
 int test_libdw_unwind(void)

tools/build/feature/test-libelf-gelf_getnote.c

···
 
 int main(void)
 {
-	return gelf_getnote(NULL, 0, NULL, NULL, NULL);
+	return gelf_getnote(NULL, 0, NULL, NULL, NULL) == 0;
 }

tools/build/feature/test-libelf.c

···
 {
 	Elf *elf = elf_begin(0, ELF_C_READ, 0);
 
-	return (long)elf;
+	return !!elf;
 }

tools/build/feature/test-lzma.c

···
 int main(void)
 {
 	lzma_stream strm = LZMA_STREAM_INIT;
-	int ret;
+	lzma_ret ret;
 
 	ret = lzma_stream_decoder(&strm, UINT64_MAX, LZMA_CONCATENATED);
 	return ret ? -1 : 0;