rtld, libthr: Use nonstring attribute to silence compiler warning

With Clang 21 at least, we start to get warnings like:

```
rtld.c:333:58: error: initializer-string for character array is too long, array size is 4 but initializer has size 5 (including the null terminating character); did you mean to use the 'nonstring' attribute? [-Werror,-Wunterminated-string-initialization]
  333 |         static const char rtld_utrace_sig[RTLD_UTRACE_SIG_SZ] = RTLD_UTRACE_SIG;
      |                                                                 ^~~~~~~~~~~~~~~
./rtld_utrace.h:49:27: note: expanded from macro 'RTLD_UTRACE_SIG'
   49 | #define RTLD_UTRACE_SIG                 "RTLD"
      |
```

Looking up the `nonstring` attribute [1], it does seem to be the correct
thing --- indicating a char array which should not have a null
terminator byte --- to use in these two cases.

Note: I discovered this when fixing Nixpkgs's FreeBSD cross compilation
support. I understand that FreeBSD may not officially support this
version of Clang yet, but I am hoping the attribute still works with the
version of Clang it does support (even if it not yet needed on that
version to fix `-Werror` errors). I am hoping thus that this patch is
accessible, as just early readiness for when FreeBSD does upgrade to
that version of Clang.

[1]: https://clang.llvm.org/docs/AttributeReference.html#nonstring

Author: Ericson2314

lib/libthr/thread/thr_printf.c

@@ -61,7 +61,7 @@ _thread_printf(int fd, const char *fmt, ...)
 void
 _thread_vprintf(int fd, const char *fmt, va_list ap)
 {
-	static const char digits[16] = "0123456789abcdef";
+	__attribute__((__nonstring__)) static const char digits[16] = "0123456789abcdef";
 	char buf[20];
 	char *s;
 	unsigned long r, u;

libexec/rtld-elf/rtld.c

@@ -348,7 +348,7 @@ ld_utrace_log(int event, void *handle, void *mapbase, size_t mapsize,
     int refcnt, const char *name)
 {
 	struct utrace_rtld ut;
-	static const char rtld_utrace_sig[RTLD_UTRACE_SIG_SZ] = RTLD_UTRACE_SIG;
+	__attribute__((__nonstring__)) static const char rtld_utrace_sig[RTLD_UTRACE_SIG_SZ] = RTLD_UTRACE_SIG;
 
 	memset(&ut, 0, sizeof(ut));	/* clear holes */
 	memcpy(ut.sig, rtld_utrace_sig, sizeof(ut.sig));