gas, arm: PR26858 Fix availability of single precision vmul/vmla in arm mode

This patch fixes a mistake when enabling MVE instructions that disabled support
for single precision vmla and vmul for arm mode.

gas/ChangeLog:
2020-11-19  Andre Vieira  <andre.simoesdiasvieira@arm.com>

	Backport from mainline.
	2020-11-12  Andre Vieira  <andre.simoesdiasvieira@arm.com>

	PR 26858
	*  config/tc-arm.c (asm_opcode insns): Fix vmul and vmla's ARM_VARIANT.
	* testsuite/gas/arm/pr26858.s: New test.
	* testsuite/gas/arm/pr26858.d: New test.