AArch64 GCS (Guarded Control Stack) is similar enough to CET that we
can re-use the existing code that is guarded by _LIBUNWIND_USE_CET,
so long as we also add defines to locate the GCS stack and pop the
entries from it. We also need the jumpto function to exit using br
instead of ret, to prevent it from popping the GCS stack.

GCS support is enabled using the LIBUNWIND_ENABLE_GCS cmake option.
This enables -mbranch-protection=standard, which enables GCS. For
the places we need to use GCS instructions we use the target
attribute, as there's not a command-line option to enable a specific
architecture extension.