Get rid of warning in linuxthreads/spinlock.c

[Roland McGrath]

> 
> this patch gets rid of:
> 
> spinlock.c:33: warning: matching constraint does not allow a register
> 
> Ok to commit?

If using the "m" constraint works right in all gcc >= 3.2, then fine.  

> 	* spinlock.c (__pthread_lock): Fix contraint to avoid warning.
					      ^typo