Provide a function in envsetup.sh to get the host arch directly

It's very simple logic, and running get_build_var to do it is slow
and has side effects (like needing a lunch target, creating an out
directory, etc.)

Test: source envsetup.sh
Change-Id: If260efd21713874fba7c15dbc0fd23442d776f8a
Merged-In: If260efd21713874fba7c15dbc0fd23442d776f8a