misc: Optimize internal usage of __libc_single_threaded

By adding an internal alias to avoid the GOT indirection.
On some architecture, __libc_single_thread may be accessed through
copy relocations and thus it requires to update also the copies
default copy.

This is done by adding a new internal macro,
libc_hidden_data_{proto,def}, which has an addition argument that
specifies the alias name (instead of default __GI_ one).

Checked on x86_64-linux-gnu and i686-linux-gnu.

Reviewed-by: Fangrui Song <maskray@google.com>

elf/libc_early_init.c

@@ -38,7 +38,7 @@ __libc_early_init (_Bool initial)
   __libc_single_threaded = initial;
 
 #ifdef SHARED
-  __libc_initial = initial;
+  __libc_single_threaded_internal = __libc_initial = initial;
 #endif
 
   __pthread_early_init ();