Add wrappers for strip commands so that files are copied to the
local drive before stripped.

Stripping files on a local folder instead of a Virtualbox shared folder
is needed to work around this bug: https://www.virtualbox.org/ticket/8463
Use absolute path to strip_wrapper because bootstrap.sh is started with
pwd /home/vagrant while vagrant up.
Merge pull request #4 from larskanis/use-wrappers-for-strip

Add wrappers for strip commands so that files are copied to the local drive before stripped.