dev-lang/go: add support for cross-compiling

dev-lang/go: add support for cross-compiling

There are a few quirks with Go to deal with when cross-compiling:

 - Since DEPEND cannot distinguish cross-architecture dependencies the
   current ebuild cannot keyword all architectures it supports. This
   could be resolved in the future with a special use flag or other
   clever trick to drop go-bootstrap from DEPEND for cross builds.

 - GOHOST* and CC variables are CBUILD values.

 - GO* and *_FOR_TARGET variables are CHOST values.

 - Go can always be used as a cross-compiler so handling the
   CHOST!=CTARGET case isn't strictly required. We could provide a
   pre-built copy of the std library for CTARGET's architecture but I'm
   not sure if that would be useful or not. For now the ebuild just
   doesn't allow that case.

 - Note from WilliamH: I made some small style changes to this commit.

This is for bug #558216.