util/u_debug: Use 'initialized' instead of 'first'

Using 'initialized' to guard the one-time init, means it can be set to
false as part of .bss instead setting 'first' to true in .data.  This
is more efficient and works at .ctor time.

Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16952>

src/util/u_debug_stack.c

@@ -301,11 +301,11 @@ static mtx_t backtrace_mutex;
 static void
 initialize_backtrace_mutex()
 {
-   static bool first = true;
+   static bool initialized = false;
 
-   if (first) {
+   if (!initialized) {
       (void)mtx_init(&backtrace_mutex, mtx_plain);
-      first = false;
+      initialized = true;
    }
 }