MIPS/GAS: Correct `mips-*-windiss' target emulation configuration

MIPS/GAS: Correct `mips-*-windiss' target emulation configuration

Fix a commit 0a44bf6950b3 ("mips-vxworks support"),
<https://sourceware.org/ml/binutils/2006-03/msg00179.html>, regression
and override the choice of the `vxworks' target environment introduced
with commit ea3eed15006f ("Add generic vxworks GAS target."),
<https://sourceware.org/ml/binutils/2005-01/msg00052.html>, for
`mips-*-windiss' targets as they have not been converted to the VxWorks
target format introduced with the former commit, removing a GAS target
format selection failure:

Assembler messages:
Fatal error: selected target format 'elf32-bigmips-vxworks' unknown

on any assembly attempt with `mips-windiss' and equivalent target
configurations.

	gas/
	* configure.tgt: Use generic emulation for `mips-*-windiss',
	overriding the blanket choice made for `*-*-windiss'.