testsuite, Darwin, PPC : XFAIL zero-scratch-regs tests.

These tests fail because of an unimplemented 'sorry'; there
is no plan to implement this in the short term, so XFAILing
the tests to reduce noise.

gcc/testsuite/ChangeLog:

	* c-c++-common/zero-scratch-regs-10.c: XFAIL for
	powerpc-darwin.
	* c-c++-common/zero-scratch-regs-11.c: Likewise.
	* c-c++-common/zero-scratch-regs-8.c: Likewise.
	* c-c++-common/zero-scratch-regs-9.c: Likewise.