[libatomic] Fix return value in libat_test_and_set

On nvptx (using a Quadro K2000 with driver 470.103.01) I ran into this:
...
FAIL: gcc.dg/atomic/stdatomic-flag-2.c -O1 execution test
...
which mimimized to:
...
  #include <stdatomic.h>
  atomic_flag a = ATOMIC_FLAG_INIT;
  int main () {
    if ((atomic_flag_test_and_set) (&a))
      __builtin_abort ();
    return 0;
  }
...

The atomic_flag_test_and_set is implemented using __atomic_test_and_set_1,
which corresponds to the "word-sized compare-and-swap loop" version of
libat_test_and_set in libatomic/tas_n.c.

The semantics of a test-and-set is that the return value is "true if and only
if the previous contents were 'set'".

But the code uses:
...
  return woldval != 0;
...
which means it doesn't look only at the byte that was either set or not set,
but at the entire word.

Fix this by using instead:
...
  return (woldval & ((UTYPE) ~(UTYPE) 0 << shift)) != 0;
...

Tested on nvptx.

libatomic/ChangeLog:

2022-03-24  Tom de Vries  <tdevries@suse.de>

	PR target/105011
	* tas_n.c (libat_test_and_set): Fix return value.

libatomic/tas_n.c

@@ -73,7 +73,7 @@ SIZE(libat_test_and_set) (UTYPE *mptr, int smodel)
 				     __ATOMIC_RELAXED, __ATOMIC_RELAXED));
 
   post_barrier (smodel);
-  return woldval != 0;
+  return (woldval & ((UTYPE) ~(UTYPE) 0 << shift)) != 0;
 }
 
 #define DONE 1