tools/nolibc: disable function sanitizer for _start_c()

Both constructors and main() may be executed with different function
signatures than they are actually using.
This is intentional but trips up UBSAN.

Disable the function sanitizer of UBSAN in _start_c().

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250419-nolibc-ubsan-v2-3-060b8a016917@weissschuh.net

tools/include/nolibc/crt.h

@@ -7,6 +7,8 @@
 #ifndef _NOLIBC_CRT_H
 #define _NOLIBC_CRT_H
 
+#include "compiler.h"
+
 char **environ __attribute__((weak));
 const unsigned long *_auxv __attribute__((weak));
 
@@ -25,6 +27,9 @@ extern void (*const __fini_array_end[])(void) __attribute__((weak));
 
 void _start_c(long *sp);
 __attribute__((weak,used))
+#if __nolibc_has_feature(undefined_behavior_sanitizer)
+	__attribute__((no_sanitize("function")))
+#endif
 void _start_c(long *sp)
 {
 	long argc;