Select MIPS ABi based upon abiflags rather than e_flags.

Select MIPS ABi based upon abiflags rather than e_flags.

bfd	PR ld/18401
	* elfxx-mips.c (bfd_mips_isa_ext_mach): New function: Converts an
	ISA value to a bfd machine number.
	(bfd_mips_isa_ext): Update the ISA level and revision if
	necessary.  Use mips_mach_extends_p to decide if the ISA extension
	needs to be set.
	(_bfd_mips_elf_merge_private_bfd_data): Allow the ISA extension to
	be inferred from the ABI setting.  Set the ISA level from the
	maximum of the incoming and outgoing ISA levels.

tests
	* ld-mips-elf/mips-elf-flags.exp: Add more ISA conflict tests.