[compiler-rt] Work around a warning from -Wgnu-anonymous-struct (#120314)

This patch works around:


compiler-rt/lib/tysan/../sanitizer_common/sanitizer_platform_limits_posix.h:604:3:
  error: anonymous structs are a GNU extension
  [-Werror,-Wgnu-anonymous-struct]

Author: kazutakahirata

compiler-rt/lib/sanitizer_common/sanitizer_platform_limits_posix.h

@@ -601,7 +601,7 @@ struct __sanitizer_siginfo_pad {
 #if SANITIZER_LINUX
 # define SANITIZER_HAS_SIGINFO 1
 union __sanitizer_siginfo {
-  struct {
+  __extension__ struct {
     int si_signo;
 # if SANITIZER_MIPS
     int si_code;