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

[Tejas Belagod]

Hi,

This patch adds support for encoding the Return Address Authentication pseudo register - '.save {ra_auth_code}' as defined by the DWARF ABI (https://github.com/ARM-software/abi-aa/releases/download/2021Q1/aadwarf32.pdf Section 4.1) - in the exception tables where the opcode is defined by the EHABI (https://github.com/ARM-software/abi-aa/releases/download/2021Q1/ehabi32.pdf Section 10.3).

Tested on arm-none-eabi. Ok for master?

Thanks,
Tejas.

gas/Changelog:

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.
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