Error for mismatched powerpc ABI tags

Error for mismatched powerpc ABI tags

And report the two input files that are incompatible rather than
reporting that an input file is incompatible with the output.

bfd/
	* elf-bfd.h (_bfd_elf_ppc_merge_fp_attributes): Update prototype.
	* elf32-ppc.c (_bfd_elf_ppc_merge_fp_attributes): Return error
	on mismatch.  Remove "warning: " from messages.  Track last bfd
	used to set tags.
	(ppc_elf_merge_obj_attributes): Likewise.  Handle status from
	_bfd_elf_ppc_merge_fp_attributes.
	* elf64-ppc.c (ppc64_elf_merge_private_bfd_data): Handle status
	from _bfd_elf_ppc_merge_fp_attributes.
ld/
	* testsuite/ld-powerpc/attr-gnu-4-12.d: Update expected output.
	* testsuite/ld-powerpc/attr-gnu-4-13.d: Likewise.
	* testsuite/ld-powerpc/attr-gnu-4-21.d: Likewise.
	* testsuite/ld-powerpc/attr-gnu-4-23.d: Likewise.
	* testsuite/ld-powerpc/attr-gnu-4-31.d: Likewise.
	* testsuite/ld-powerpc/attr-gnu-4-32.d: Likewise.
	* testsuite/ld-powerpc/attr-gnu-8-23.d: Likewise.
	* testsuite/ld-powerpc/attr-gnu-12-21.d: Likewise.