[ARM] Make human parsing of "processor does not support instruction in mode" error...

[ARM] Make human parsing of "processor does not support instruction in mode" error messages easier

2015-07-16  James Greenhalgh  <james.greenhalgh@arm.com>

      gas/
	* config/tc-arm.c (md_assemble): Rephrase the "selected processor does
	not support ARM mode" error messages.

      gas/testsuite/
	* gas/arm/arch7em-bad.l: Update expected errors.
	* gas/arm/arch7m-bad.l: Likewise.
	* gas/arm/arm-idiv-bad.l: Likewise.
	* gas/arm/arm7-bad.l: Likewise.
	* gas/arm/armv1-bad.l: Likewise.
	* gas/arm/thumb-w-bad.l: Likewise.