Add chroot version hook to unmerge syslibs/libunwind

It has been replaced by sys-libs/llvm-libunwind and needs to be
unmerged. Otherwise, setup_board fails.

BUG=None
TEST=Ran hooks

Change-Id: I454afa652adaf1228e1fe8bb575548274254403d
Reviewed-on: https://chromium-review.googlesource.com/1188282
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
1 file changed