arm64: add CONTEXTIDR_EL1 reg

CONTEXTIDR_EL1 is used in debug and trace features to identify the
current process or context.

Reviewed by: andrew
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D45173

(cherry picked from commit 4f8ba1c9dd00e0ae20fa161e166715746b6d1c04)