MIPS/BFD: Factor out ELF file header flag checks

MIPS/BFD: Factor out ELF file header flag checks

Factor out the parts of `_bfd_mips_elf_merge_private_bfd_data'
responsible for ELF file header flag compatibility checks to
`mips_elf_merge_obj_e_flags'.  As a side effect remove a premature
return point from `_bfd_mips_elf_merge_private_bfd_data'.  No functional
change otherwise.

	bfd/
	* elfxx-mips.c (mips_elf_merge_obj_e_flags): New function,
	factored out from...
	(_bfd_mips_elf_merge_private_bfd_data): ... here.