Implement dtrace_getupcstack in ARM64

Allow using DTRACE for performance analysis of userspace
applications - the function call stack can be captured.
This is almost an exact copy of AMD64 solution.

Obtained from: Semihalf
Sponsored by: Cavium
Reviewed by: emaste, gnn, jhibbits
Differential Revision: https://reviews.freebsd.org/D5779