host-go: new package

This is the golang compiler which allows cross compilation

Here a basic example, with HOST=386 and target=arm

    $ make host-go
    ...
    $ $(HOST_DIR)/usr/bin/go version
    go version go1.3 linux/386
    $ cat hello.go
    package main
    import "fmt"
    func main() {
	fmt.Println("Hello, world")
    }
    $ $(HOST_DIR)/usr/bin/go build hello.go
    $ file hello
    hello: ELF 32-bit LSB  executable, Intel 80386, version 1 (SYSV), statically linked, not stripped
    $ ./hello
    Hello, world
    $ GOARCH=arm GOARM=5 $(HOST_DIR)/usr/bin/go build hello.go
    $ file hello
    hello: ELF 32-bit LSB  executable, ARM, EABI5 version 1 (SYSV), statically linked, not stripped