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>

misc/single_threaded.c

@@ -25,3 +25,5 @@ char __libc_single_threaded;
 #else
 char __libc_single_threaded = 1;
 #endif
+libc_hidden_data_def_alias (__libc_single_threaded,
+			    __libc_single_threaded_internal)