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

[Nick Clifton]

Hi Vladimir,

> gas/ChangeLog:
> 
> 2019-11-27  Vladimir Murzin <vladimir.murzin@arm.com>
> 
> 	* config/tc-arm.c (s_arm_arch): Set selected_ctx_ext_table.
> 	* gas/testsuite/gas/arm/mve-arch-ext.s: New.
> 	* gas/testsuite/gas/arm/mve-arch-ext.d: New.

Approved - please apply.

Cheers
  Nick