[Xtensa] revert change to test synchronization builtins

[Bob Wilson]

This patch reverts my previous change to enable testing  
synchronization builtins on Xtensa.  I had forgotten that not all  
Xtensa processors can support this feature.  Committed on mainline.

2007-09-12  Bob Wilson  <bob.wilson@acm.org>

         * lib/target-supports.exp  
(check_effective_target_sync_int_long):
         Revert change to add xtensa.
         (check_effective_target_sync_char_short): Likewise.