[Patch, Arm, Gas] Add unwinder encoding support for PACBTI.

[Nick Clifton]

Hi Tejas,

> 2021-10-05  Tejas Belagod  <tejas.belagod@arm.com>
> 
> 	* config/tc-arm.c (arm_reg_type): Add new type REG_TYPE_PSEUDO.
> 	(reg_expected_msgs): Add message for pseudo reg type.
> 	(reg_list_els): Add new reg list type REGLIST_PSEUDO.
> 	(parse_reg_list): Handle new REGLIST_PSEUDO type.
> 	(s_arm_unwind_save_pseudo): Encode pseudo reg list save in exception
> 	tables.
> 	(s_arm_unwind_save): Handle new REG_TYPE_PSEUDO.
> 	(reg_names): Add ra_auth_code pseudo register.
> 	* testsuite/gas/arm/unwind-pacbti-m.s: New test.
> 	* testsuite/gas/arm/unwind-pacbti-m.d: New test.
> 	* testsuite/gas/arm/unwind-pacbti-m-readelf.d: New test.

Approved - please apply.

Cheers
   Nick