arm64: Use the Linux sigframe to restore registers

When returning from a Linux signal use the Linux sigframe to find the
register values to restore.

Remove the FreeBSD ucontext from the stack as it's now unneeded.

Reviewed by: dchagin, emaste
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D42360

(cherry picked from commit 070a4ff82a34652d533f9315ae9ad0aa8f1fdeb2)