Fix unwanted sign-extension of ID register masks. Sign-extension of the

Fix unwanted sign-extension of ID register masks.  Sign-extension of the
GPI feature mask caused misdetection of the GPI feature when some other
feature was present that was advertised in the upper 32 bits of the same
ID register.  Resulting in a crash as soon as the pmap code tried to set
the PAC keys.

Fix suggested by Marc Zyngier who found and debugged the problem.

ok jsg@, deraadt@