PowerPC nops

PowerPC nops

This patch corrects ppc rs_align_code handling to choose the alignment
nops based on the machine in force at the alignment directive rather
than the machine at the end of file.

	* config/tc-ppc.h (ppc_nop_select): Declare.
	(NOP_OPCODE): Define.
	* config/tc-ppc.c (ppc_elf_end, ppc_xcoff_end): Zero ppc_cpu.
	(ppc_nop_encoding_for_rs_align_code): New enum.
	(ppc_nop_select): New function.
	(ppc_handle_align): Don't use ppc_cpu here.  Get nop type from frag.
	* testsuite/gas/ppc/groupnop.d,
	* testsuite/gas/ppc/groupnop.s: New test.
	* testsuite/gas/ppc/ppc.exp: Run it.