[lld][AArch64] Add support for GCS (#90732)

This adds the -z gcs and -z gcs-report options, which behave similarly
to -z shtk and -z cet-report, except that -z gcs accepts a parameter:
* -z gcs=implicit is the default behaviour, where the GCS bit is
inferred from the input objects.
 * -z gcs=never clears the GCS bit, ignoring the input objects.
 * -z gcs=always sets the GCS bit, ignoring the input objects.

This is so that there's a means of explicitly disabling GCS even when
all input objects have the GCS bit set.