nptl: fix gcc noreturn warning

Current builds show the warning:
  CC libpthread/nptl/pthread_create.os
libpthread/nptl/pthread_create.c: In function 'start_thread':
libpthread/nptl/pthread_create.c:418:3: warning: function declared
	'noreturn' has a 'return' statement [enabled by default]
libpthread/nptl/pthread_create.c:418:3: warning: 'noreturn' function
	does return [enabled by default]

Looking at glibc, it does not mark this function as noreturn.  Testing
code size, gcc is smart enough to detect the code never actually returns
regardless of the attribute.  So drop it to keep in sync with glibc and
to avoid the warnings.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

libpthread/nptl/pthread_create.c

@@ -221,7 +221,7 @@ __free_tcb (struct pthread *pd)
 }
 
 
-static int attribute_noreturn
+static int
 start_thread (void *arg)
 {
   struct pthread *pd = (struct pthread *) arg;