[gas][arm] Set context table for '.arch_extension'

[gas][arm] Set context table for '.arch_extension'

This patch fixes .arch_extension behaviour.

Currently, context table for '.arch_extension' is only set while
"-march" processing, but it is not set while .arch processing,
so following code is rejected

  .syntax unified
  .thumb
  .arch armv8.1-m.main
  .arch_extension mve.fp

  unless -march=armv8.1-m.main is given.

Committing on behalf of Vladimir Murzin

gas/ChangeLog:
2019-12-10  Vladimir Murzin  <vladimir.murzin@arm.com>

	* config/tc-arm.c (s_arm_arch): Set selected_ctx_ext_table.
	* testsuite/gas/arm/mve-arch-ext.s: New.
	* testsuite/gas/arm/mve-arch-ext.d: New.