git_bootstrap.py: add a switch to control client bitness
git_bootstrap.py: add a switch to control client bitness

BUG=635421, 629679

Review-Url: https://codereview.chromium.org/2280973002

primiano/depot_tools,primiano/depot_tools,CoherentLabs/depot_tools,primiano/depot_tools,CoherentLabs/depot_tools