Allow optional override of os and arch in build

With this change machine will still cross-compile for all supported OSes
/ archs by default, but allows user to customize args to the build
script.

Signed-off-by: Nathan LeClaire <nathan.leclaire@gmail.com>
GCE Driver.

Signed-off-by: Dan Lorenc <lorenc.d@gmail.com>
Signed-off-by: dlorenc <lorenc.d@gmail.com>