Add support for context sensitive '.arch_extension' to the ARM assembler.

Add support for context sensitive '.arch_extension' to the ARM assembler.

If the extension is not found in the context sensitive table, the legacy
tables are still checked as a fallback.  This is particularly useful for
Armv8.1-M as it enables  the use of '.arch_extension' with the 'mve' and
'mve.fp' extensions which are not part of the legacy table.

	* config/tc-arm.c (selected_ctx_ext_table) New static variable.
	(arm_parse_arch): Set context sensitive extension table based on the
	chosen base architecture.
	(s_arm_arch_extension): Change to lookup extensions in the new context
	sensitive tables.
	* gas/testsuite/gas/arm/mve-ext.s: New.
	* gas/testsuite/gas/arm/mve-ext.d: New.
	* gas/testsuite/gas/arm/mvefp-ext.s: New.
	* gas/testsuite/gas/arm/mvefp-ext.d: New.