testsuite: skip zero-scratch-regs on powerpc.

These tests are unsupported on PowerPC.

gcc/testsuite/ChangeLog:

	* c-c++-common/zero-scratch-regs-10.c: Skip on powerpc*-*-*.
	* c-c++-common/zero-scratch-regs-11.c: Skip on powerpc*-*-*.
	* c-c++-common/zero-scratch-regs-5.c: Skip on powerpc*-*-aix*.
	* c-c++-common/zero-scratch-regs-8.c: Skip on powerpc*-*-*.
	* c-c++-common/zero-scratch-regs-9.c: Skip on powerpc*-*-*.