_GOPATH incorrectly set for cross-compiled go_library targets

[katzdm]

By defining _GOPATH here, the variable seems to pick up the host CONFIG.OS and CONFIG.ARCH. The result is that setting --arch does not override the paths in _GOPATH; builds subsequently fail.

This appears to be fixable by inlining the definition of _GOPATH into _go_library_cmds and _go_binary_cmds. I'll put a PR together in a bit, if nobody beats me to it first :-P