aarch64: Add FEAT_ECBHB support

This patch add support for FEAT_ECBHB "Exploitative control using
branch history information" adding the "clrbhb" instruction.  AFAIU
the same alias was originally added as "clearbhb" before the
architecture was finalized (Mandatory v8.9-a/v9.4-a; Optional
v8.0-a+/v9.0-a+).