DTrace aarch64: Avoid calling unwind_frame() in the probe context.

unwind_frame() may be instrumented by FBT, leading to recursion into
dtrace_probe(). Manually inline unwind_frame() as we do with stack
unwinding code for other architectures.

Submitted by: Domagoj Stolfa
Reviewed by: manu
MFC after: 1 week
Sponsored by: DARPA / AFRL
Differential Revision: https://reviews.freebsd.org/D15359