configure: respect CC_host in host arch detection

When cross compiling, GYP uses the variables CC_host and CXX_host to
find the host compiler, if they are defined. This ensures that
variable is used, if defined, when detecting the host architecture.

Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/4117