selftests/bpf: Fix compile if backtrace support missing in libc

Include GNU <execinfo.h> header only with glibc and provide weak, stubbed
backtrace functions as a fallback in test_progs.c. This allows for non-GNU
replacements while avoiding compile errors (e.g. with musl libc) like:

  test_progs.c:13:10: fatal error: execinfo.h: No such file or directory
     13 | #include <execinfo.h> /* backtrace */
        |          ^~~~~~~~~~~~
  test_progs.c: In function 'crash_handler':
  test_progs.c:1034:14: error: implicit declaration of function 'backtrace' [-Werror=implicit-function-declaration]
   1034 |         sz = backtrace(bt, ARRAY_SIZE(bt));
        |              ^~~~~~~~~
  test_progs.c:1045:9: error: implicit declaration of function 'backtrace_symbols_fd' [-Werror=implicit-function-declaration]
   1045 |         backtrace_symbols_fd(bt, sz, STDERR_FILENO);
        |         ^~~~~~~~~~~~~~~~~~~~

Fixes: 9fb156b ("selftests/bpf: Print backtrace on SIGSEGV in test_progs")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/aa6dc8e23710cb457b278039d0081de7e7b4847d.1722244708.git.tony.ambardar@gmail.com

tools/testing/selftests/bpf/test_progs.c

@@ -10,7 +10,6 @@
 #include <sched.h>
 #include <signal.h>
 #include <string.h>
-#include <execinfo.h> /* backtrace */
 #include <sys/sysinfo.h> /* get_nprocs */
 #include <netinet/in.h>
 #include <sys/select.h>
@@ -19,6 +18,21 @@
 #include <bpf/btf.h>
 #include "json_writer.h"
 
+#ifdef __GLIBC__
+#include <execinfo.h> /* backtrace */
+#endif
+
+/* Default backtrace funcs if missing at link */
+__weak int backtrace(void **buffer, int size)
+{
+	return 0;
+}
+
+__weak void backtrace_symbols_fd(void *const *buffer, int size, int fd)
+{
+	dprintf(fd, "<backtrace not supported>\n");
+}
+
 static bool verbose(void)
 {
 	return env.verbosity > VERBOSE_NONE;