util: Add a helper macro for defining initial-exec variables.

I'm going to add another case for Android shortly, and then we can keep
the logic all in one spot.

Reviewed-by: Roman Stratiienko <r.stratiienko@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10389>

src/egl/main/eglcurrent.c

@@ -32,6 +32,7 @@
 #include <stdarg.h>
 #include "c99_compat.h"
 #include "c11/threads.h"
+#include "util/u_thread.h"
 
 #include "egllog.h"
 #include "eglcurrent.h"
@@ -45,8 +46,7 @@ static tss_t _egl_TSD;
 static void _eglDestroyThreadInfo(_EGLThreadInfo *t);
 
 #ifdef USE_ELF_TLS
-static __thread const _EGLThreadInfo *_egl_TLS
-   __attribute__ ((tls_model("initial-exec")));
+static __THREAD_INITIAL_EXEC const _EGLThreadInfo *_egl_TLS;
 #endif
 
 static inline void _eglSetTSD(const _EGLThreadInfo *t)