[gas] arm: Add support for new unwinder directive ".pacspval".

[gas] arm: Add support for new unwinder directive ".pacspval".

This patch adds the assembler support for the new unwinder
directive ".pacspval" and encodes this directives with opcode
"0xb5". This opcode indicates the unwinder to use effective
vsp as modifier for PAC validation.

gas/ChangeLog:

2022-11-07  Srinath Parvathaneni  <srinath.parvathaneni@arm.com>

        * doc/c-arm.texi: Document directive.
        * config/tc-arm.c (s_arm_unwind_pacspval): Define function.
        (md_pseudo_table): Add entry for pacspval directive.
        * testsuite/gas/arm/ehabi-pacbti-m.d: New test.
        * testsuite/gas/arm/ehabi-pacbti-m.s: Likewise.