MIPS/BFD: Fold the handling of input MIPS ABI flags together

MIPS/BFD: Fold the handling of input MIPS ABI flags together

Fold the handling of input MIPS ABI flags in private BFD data merging
together, moving the attribute check afterwards, and consequently making
input vs output compatibility checks only start once all input ABI flag
data has been gathered, checked for inconsistencies and put in order.

Consequently also address the issue of input ABI flag inconsistencies
being masked by a failing attribute check, which currently makes
`_bfd_mips_elf_merge_private_bfd_data' exit prematurely and therefore
prevent input ABI flag inconsistencies from being reported.  Such
inconsistencies need to be reported as they may be the very cause of an
attribute check failure.

	bfd/
	* elfxx-mips.c (_bfd_mips_elf_merge_private_bfd_data): Fold the
	handling of input MIPS ABI flags together.