Provide a function in envsetup.sh to get the host arch directly am: 32b2aa33

Original change: https://android-review.googlesource.com/c/platform/build/+/3086688



Change-Id: I4ff8e19005b71412027c5c0ff191b5d5a509709f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>