[libunwind] Add support for PC reg column in arm64

This change adds support for the dwarf PC register column in arm64, allowing
CFI directives to make use of it.

As of the last revision of the DWARF for ARM 64-bit architecture[0], the pc
register has been added as a valir register, with number 32.

This allows libunwinder to restore both pc and lr, which is useful
for stack switches and signal contexts.

[0]:
https://github.com/ARM-software/abi-aa/blob/f52e1ad3f81254497a83578dc102f6aac89e52d0/aadwarf64/aadwarf64.rst

Reviewed By: phosek, #libunwind

Differential Revision: https://reviews.llvm.org/D96901