Determine int, uint and uintptr bit sizes from GOARCH environment
variable if it is set. Otherwise use host-specific sizes.

Fixes #19321

Change-Id: I494b8e4b49b59d32794f50ff2ce06ba040cb8460
Reviewed-on: https://go-review.googlesource.com/37950
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>