[gas][arm] Enable VLDM, VSTM, VPUSH, VPOP for MVE

[gas][arm] Enable VLDM, VSTM, VPUSH, VPOP for MVE

This patch enables a few instructions for Armv8.1-M MVE. Currently VLDM,
VSTM, VSTR, VLDR, VPUSH and VPOP are enabled only when the Armv8-M
Floating-point Extension is enabled.  According to the ARMv8.1-M ARM,
section A.1.4.2[1], they can be enabled by having "Armv8-M Floating-point
Extension and/or Armv8.1-M MVE".

[1]https://developer.arm.com/docs/ddi0553/bh/armv81-m-architecture-reference-manual

2019-11-12  Mihail Ionescu  <mihail.ionescu@arm.com>

	* config/tc-arm.c (do_vfp_nsyn_push): Move in order to enable it for
	both fpu_vfp_ext_v1xd and mve_ext and add call to the aliased vstm
	instruction for mve_ext.
	(do_vfp_nsyn_pop): Move in order to enable it for both
	fpu_vfp_ext_v1xd and mve_ext and add call to the aliased vldm
	instruction for mve_ext.
	(do_neon_ldm_stm): Add fpu_vfp_ext_v1 and mve_ext checks.
	(insns): Enable vldm, vldmia, vldmdb, vstm, vstmia, vstmdb, vpop,
	vpush, and fldd, fstd, flds, fsts for arm_ext_v6t2 instead
	of fpu_vfp_ext_v1xd.
	* testsuite/gas/arm/v8_1m-mve.s: New.
	* testsuite/gas/arm/v8_1m-mve.d: New.