MIPS/GAS: Clear the ASE_MIPS16E2_MT flag for recalculation

MIPS/GAS: Clear the ASE_MIPS16E2_MT flag for recalculation

Correct a commit 25499ac7ee92 ("MIPS16e2: Add MIPS16e2 ASE support") GAS
bug with the handling of the ASE_MIPS16E2_MT combination ASE flag, which
is not correctly calculated as `.set nomips16e2' and `.set nomt'
pseudo-ops are processed.  This leads to code like:

$ cat foo.s
	.set	nomt
	evpe
	.align	4, 0
$ cat bar.s
	.set	nomips16e2
	dvpe
	.align	4, 0
$

to successfully assemble where it should not:

$ as -32 -mips32r3 -mmt -mips16 -mmips16e2 -o foo.o foo.s
$ as -32 -mips32r3 -mmt -mips16 -mmips16e2 -o bar.o bar.s
$ objdump -m mips:16 -d foo.o

foo.o:     file format elf32-tradbigmips

Disassembly of section .text:

00000000 <.text>:
   0:	f027 6700 	evpe
	...

bar.o:     file format elf32-tradbigmips

Disassembly of section .text:

00000000 <.text>:
   0:	f026 6700 	dvpe
	...
$

This happens because ASE_MIPS16E2_MT once set in `mips_set_ase' is never
cleared.  Fix the problem by clearing it there before it is calculated
based on the ASE_MT and ASE_MIPS16E2 flags, making assembly fail as
expected:

$ as -32 -mips32r3 -mmt -mips16 -mmips16e2 -o foo.o foo.s
foo.s: Assembler messages:
foo.s:2: Error: opcode not supported on this processor: mips32r3 (mips32r3) `evpe'
$ as -32 -mips32r3 -mmt -mips16 -mmips16e2 -o bar.o bar.s
bar.s: Assembler messages:
bar.s:2: Error: opcode not supported on this processor: mips32r3 (mips32r3) `dvpe'
$

	gas/
	* config/tc-mips.c (mips_set_ase): Clear the ASE_MIPS16E2_MT
	flag before recalculating.
	* testsuite/gas/mips/mips16e2-mt-err.d: New test.
	* testsuite/gas/mips/mips16e2-mt-err.l: New stderr output.
	* testsuite/gas/mips/mips16e2-mt-err.s: New test source.
	* testsuite/gas/mips/mips.exp: Run the new test.