GDB: aarch64: Use GCS features to calculate hash of struct aarch64_features

GDB: aarch64: Use GCS features to calculate hash of struct aarch64_features

Luis noticed that when adding the gcs and gcs_linux members to struct
aarch64_features in my Guarded Control Stack patch series, I neglected to
modify struct hash<aarch64_features>::operator() to take them into account
when computing its hash.

This can cause GDB to use the wrong aarch64_features object during a
debugging session.

Regression tested on aarch64-linux-gnu.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33440
Suggested-by: Luis Machado <luis.machado.foss@gmail.com>
Approved-By: Luis Machado <luis.machado.foss@gmail.com>