Sync a few changes with the upstream one, notably get_toolchain_if_necessary.py
needs GYP_MSVS_VERSION set. Also pull the variables that change up to the top.
This diverges a bit more from the upstream one, but we're already heavily
diverged. If we ever need to support two concurrent toolchains, I'll bring us
closer to parity.

Change-Id: I6db7fbaccd5dddd92ad2deee15bd6dd3e28841f7
Reviewed-on: https://boringssl-review.googlesource.com/7830
Reviewed-by: Steven Valdez <svaldez@google.com>
Reviewed-by: David Benjamin <davidben@google.com>