Because the first stage for building target Go is actually a host build,
the default platform options (GO386, GOARM, etc.) are detected from the
host. These values are written to a source file and kept when building
the second stage.

This modifies this source file to set the appropriate values for the
target platform, and reset values for other platforms to their
cross-compiling / most compatible defaults.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>