MIPS/GAS: Correct branch relaxation for weak symbols

MIPS/GAS: Correct branch relaxation for weak symbols

Weak symbols can be preempted at link time so always choose the longer
sequence in branch relaxation, according to the relaxation level chosen,
so that any symbol finally used as the branch target is reachable.

2016-04-13  Maciej W. Rozycki  <macro@imgtec.com>
            Andrew Bennett  <andrew.bennett@imgtec.com>

	gas/
	* config/tc-mips.c (relaxed_branch_length): Use the long
	sequence where the target is a weak symbol.
	(relaxed_micromips_32bit_branch_length): Likewise.
	(relaxed_micromips_16bit_branch_length): Likewise.
	* testsuite/gas/mips/branch-weak-1.d: New test.
	* testsuite/gas/mips/branch-weak-2.d: New test.
	* testsuite/gas/mips/branch-weak-3.d: New test.
	* testsuite/gas/mips/branch-weak-4.d: New test.
	* testsuite/gas/mips/branch-weak-5.d: New test.
	* testsuite/gas/mips/branch-weak.l: New stderr output.
	* testsuite/gas/mips/branch-weak.s: New test source.
	* testsuite/gas/mips/mips.exp: Run the new tests.