Fix value for O_mask_was_saved based on gdb observation

(gdb) print offsetof(struct __jmp_buf_tag, __mask_was_saved)
$12 = (int *) 0x1f0

using https://stackoverflow.com/a/39663128/2171120

libc/sysdeps/linux/sparc64/jmpbuf-offsets.h

@@ -15,6 +15,6 @@
    License along with the GNU C Library; if not, see
    <http://www.gnu.org/licenses/>.  */
 
-#define O_mask_was_saved	512
+#define O_mask_was_saved	496
 #define O_gregs			32
 #define O_g1			(O_gregs + 4*8)