MIPS16/GAS: Improve disallowed relocation operand error diagnostics

MIPS16/GAS: Improve disallowed relocation operand error diagnostics

Improve disallowed relocation operand error diagnostics for MIPS16 code
and make it match corresponding regular MIPS and microMIPS handling,
e.g:

$ cat sltu.s
	sltu	$2, %lo(foo)
$ as -o sltu.o sltu.s
sltu.s: Assembler messages:
sltu.s:1: Error: operand 2 must be constant `sltu $2,%lo(foo)'
$ as -mips16 -o sltu.o sltu.s
sltu.s: Assembler messages:
sltu.s:1: Error: invalid operands `sltu $2,%lo(foo)'
$

To do so call `match_not_constant' from `match_mips16_insn' whenever a
disallowed relocation operation has been noticed, like `match_const_int'
does, making reporting consistent:

$ as -mips16 -o sltu.o sltu.s
sltu.s: Assembler messages:
sltu.s:1: Error: operand 2 must be constant `sltu $2,%lo(foo)'
$

	gas/
	* config/tc-mips.c (match_mips16_insn): Call
	`match_not_constant' for a disallowed relocation operation.
	* testsuite/gas/mips/mips16-reloc-error.d: New test.
	* testsuite/gas/mips/mips16-reloc-error.l: New stderr output.
	* testsuite/gas/mips/mips16-reloc-error.s: New test source.
	* testsuite/gas/mips/mips.exp: Run the new test.