MIPS16/GAS: Fix assertion failures with relocations on 16-bit instructions

MIPS16/GAS: Fix assertion failures with relocations on 16-bit instructions

Complement commit c9775dde3277 ("MIPS16: Add R_MIPS16_PC16_S1 branch
relocation support)" and report an assembly error when a relocation is
required for an instruction, currently a branch only, that has been
forced to use its unextended encoding, either with the use of an
explicit `.t' mnemonic suffix, or by means of `.set noautoextend' being
active, fixing an assertion failure currently caused instead.

	gas/
	* config/tc-mips.c (md_convert_frag): Report an error instead of
	asserting on `ext'.
	* testsuite/gas/mips/mips16-branch-unextended-1.d: New test.
	* testsuite/gas/mips/mips16-branch-unextended-2.d: New test.
	* testsuite/gas/mips/mips16-branch-unextended-1.s: New test
	source.
	* testsuite/gas/mips/mips16-branch-unextended-2.s: New test.
	* testsuite/gas/mips/mips16-branch-unextended.l: New stderr
	output.
	* testsuite/gas/mips/mips.exp: Run the new tests.