[PAC][libunwind] Enhance PAuth-related comments in `__unw_set_reg` (#173769)

The PAuth-related checks performed for arm64e are also performed for
other PAuth-enabled ABIs such as Linux's pauthtest. This patch
introduces use of 'PAuth-enabled ABI' phrase instead of 'arm64e' term
for non-arm64e specific things. Also, on non-arm64e we cannot assume
that we have FPAC, so an additional explanation regarding resign
function guarantees is provided.

Note that other comments mentioning arm64e while not being
arm64e-specific are fixed as part of #171717.

Fixes #160117

GitOrigin-RevId: 367e3889fabcc3dcd3ee2d664d652ab6ec26854b