arm: add unwinder encoding support for PACBTI

This patch adds support for encoding the Return Address Authentication pseudo
register - '.save {ra_auth_code}' as defined by the DWARF ABI - in the
exception tables where the opcode is defined by the EHABI

gas/Changelog:

	* 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.