MIPS/GAS: Keep the original microMIPS symbol reference in branch relocs

MIPS/GAS: Keep the original microMIPS symbol reference in branch relocs

Keep original microMIPS symbols in references from branch relocations so
that the ISA bit is retained and can be verified for validity in static
link.  No need to update WRT MIPS16 symbols because we keep them all
anyway for other reasons.

	gas/
	* config/tc-mips.c (b_reloc_p): New function.
	(mips_fix_adjustable): Also keep the original microMIPS symbol
	referred from branch relocations.
	* testsuite/gas/mips/branch-local-1.d: New test.
	* testsuite/gas/mips/branch-local-n32-1.d: New test.
	* testsuite/gas/mips/branch-local-n64-1.d: New test.
	* testsuite/gas/mips/micromips@branch-misc-4-64.d: Update
	relocations.
	* testsuite/gas/mips/branch-local-1.s: New test source.
	* testsuite/gas/mips/mips.exp: Run the new cases.