[ASan] Reenable __builtin_setjmp test on PowerPC, disable on SystemZ.

Since __builtin_setjmp has been fixed by rL267943, the test now works
on PowerPC.  Enable it.

On the other hand, the SystemZ backend doesn't currently support
__builtin_setjmp.  Disable it.

Differential Revision: http://reviews.llvm.org/D19657

llvm-svn: 267946