arm: Add cfi expression support for ra_auth_code

This patch extends assembler support for the use of register names to
allow for pseudo-registers, e.g. ra_auth_code register.
This is done particularly with CFI directives in mind, allowing for
expressions of the type:

    .cfi_register ra_auth_code, 12

gas/Changelog:

	* config/tc-arm.c (tc_arm_regname_to_dw2regnum): Add
	REG_TYPE_PSEUDO handling.
	* testsuite/gas/arm/cfi-pacbti-m-readelf.d: New.
	* testsuite/gas/arm/cfi-pacbti-m.s: New.