Fix unw_getcontext on __arm64__

It looks like this was accidentally changed in the git rebasing. The
code here is identical to what's in the Restore.S file (because these
two files used to have their contents flipped 8 months ago.)

Replace the function with what's in upstream.

Change-Id: I95b1cb1c80a1d11dd0cbe5e78fd0c564c11165d4