[AArch64] Return the regnum for PC (32) on aarch64

This will allow the unwind info to explicitly specify a different value
for the return address from the link register.
Such usage, although uncommon, is valid and useful for signal frames.
It is also supported by aadwarf64 from ARM (Note 9 in [1]).

Ref https://sourceware.org/pipermail/gdb/2022-May/050091.html

[1] https://github.com/ARM-software/abi-aa/blob/2022Q1/aadwarf64/aadwarf64.rst#dwarf-register-names

Signed-off-by: Luis Machado <luis.machado@arm.com>