fix(pauth): make pauth_helpers linking generic

Pauth is a generic Arm feature that can be enabled on any platform that
implements it. It only needs a platform specific key generation hook. As
such, the generic Pauth enablement can be included in the generic build.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: Ibf32f79addab3515214594bb8d7168151b450f59