libc/aarch64: Add a non-trivial getcontextx

Add support for extra registers to the arm64 getcontextx. As no
registers are defined this just adds the extra register list and an end
context.

Reviewed by: kib
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D44255

(cherry picked from commit 5202ca77aaf552310bcbaccba750ee9f0888d0d8)