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
Merge pull request #152 from awong-dev/asmsnafu

Fix unw_getcontext on __arm64__