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>
Merge pull request #106 from nathanleclaire/modify_osarch

Allow optional override of OS and arch in build (speed up compilation)