gh-129838: Don't redefine _Py_NO_SANITIZE_UNDEFINED (#129839)

Newer GCC versions accept both __attribute__((no_sanitize("undefined")))
and __attribute__((no_sanitize_undefined)) so check that the macro is
not already defined.

Author: collinfunk

Misc/NEWS.d/next/Build/2025-02-07-21-20-21.gh-issue-129838.fkuiEc.rst

@@ -0,0 +1,2 @@
+Don't redefine ``_Py_NO_SANITIZE_UNDEFINED`` when compiling with a recent
+GCC version and undefined sanitizer enabled.

Modules/faulthandler.c

@@ -37,14 +37,15 @@
 #define PUTS(fd, str) (void)_Py_write_noraise(fd, str, strlen(str))
 
 
-// clang uses __attribute__((no_sanitize("undefined")))
-// GCC 4.9+ uses __attribute__((no_sanitize_undefined))
-#if defined(__has_feature)  // Clang
+// Clang and GCC 9.0+ use __attribute__((no_sanitize("undefined")))
+#if defined(__has_feature)
 #  if __has_feature(undefined_behavior_sanitizer)
 #    define _Py_NO_SANITIZE_UNDEFINED __attribute__((no_sanitize("undefined")))
 #  endif
 #endif
-#if defined(__GNUC__) \
+
+// GCC 4.9+ uses __attribute__((no_sanitize_undefined))
+#if !defined(_Py_NO_SANITIZE_UNDEFINED) && defined(__GNUC__) \
     && ((__GNUC__ >= 5) || (__GNUC__ == 4) && (__GNUC_MINOR__ >= 9))
 #  define _Py_NO_SANITIZE_UNDEFINED __attribute__((no_sanitize_undefined))
 #endif