[GAS, ARM, 2/16] Add CLI extension support for Armv8.1-M Mainline

[GAS, ARM, 2/16] Add CLI extension support for Armv8.1-M Mainline

This patch implements the dsp, fp and fp.dp extensions for Armv8.1-M Mainline.

This patch also removes the fp-armv8 check from the half-precision move
instructions 'do_neon_movhf', as checking that the FP16 instructions extension
feature bit is enabled 'ARM_EXT2_FP16_INST' is enough.

gas/ChangeLog:
2019-04-15  Andre Vieira  <andre.simoesdiasvieira@arm.com>

	* config/tc-arm.c (do_neon_movhf): Remove fp-armv8 check.
	(armv8_1m_main_ext_table): New extension table.
	(arm_archs): Use the new extension table.
	* doc/c-arm.texi: Add missing arch and document new extensions.
	* testsuite/gas/arm/armv8.1-m.main-fp.d: New.
	* testsuite/gas/arm/armv8.1-m.main-fp-dp.d: New.
	* testsuite/gas/arm/armv8.1-m.main-hp.d: New.