[ARM]: Add support for vldmia/vldmdb/vstmia/vstmdb instructions in MVE.

[ARM]: Add support for vldmia/vldmdb/vstmia/vstmdb instructions in MVE.

This patch adds support for assembly instructions vldmia, vldmdb, vstmia
and vstmdb in MVE.  This instructions are already supported for Armv8-M
Floating-point Extension.

gas/ChangeLog:

2020-01-31  Srinath Parvathaneni  <srinath.parvathaneni@arm.com>

	* config/tc-arm.c (fldmias): Moved inside "THUMB_VARIANT & arm_ext_v6t2"
	to support VLDMIA instruction for MVE.
	(fldmdbs): Moved inside "THUMB_VARIANT & arm_ext_v6t2" to support VLDMDB
	instruction for MVE.
	(fstmias): Moved inside "THUMB_VARIANT & arm_ext_v6t2" to support VSTMIA
	instruction for MVE.
	(fstmdbs): Moved inside "THUMB_VARIANT & arm_ext_v6t2" to support VSTMDB
	instruction for MVE.
	* testsuite/gas/arm/mve-ldst.d: New test.
	* testsuite/gas/arm/mve-ldst.s: Likewise.