Go on Fuchsia

To build a toolchain on Linux for Fuchsia, first follow the instructions on

https://fuchsia.googlesource.com/manifest/

Then,

Set the SDK environment variable to the location of the Fuchsia SDK: If you downloaded fuchsia to your ${HOME} directory:
```
export SDK=${HOME}/fuchsia/buildtools
```
Identify where a host version of go exists, with:
```
export GOROOT_BOOTSTRAP=`path to directory containing go`
```
On my machine, which go returned /usr/lib/golang/bin/go, so I set GOROOT_BOOTSTRAP=/usr/lib/golang/

Build Go in fuchsia/third_party/go/src using

GOOS=fuchsia CC_FOR_TARGET=../misc/fuchsia/clangwrap.sh ./make.bash

To build a program, junk.go in go/src, use:

GOOS=fuchsia ../bin/go build junk.go

This program can be put in a bootfs and run on qemu with:

echo bin/junk=junk > junk.manifest
fuchsia/buildtools/mkbootfs -o junk.bootfs junk.manifest
./scripts/run-magenta-x86-64 -x path/to/fuchsia/third_party/go/src/junk.bootfs

A cgo variant can be build by adding CGO_ENABLED=1 to both the make.bash command and the go build command.

To build a program using just the magenta prebuild toolchain (useful for debugging if the fuchsia sysroot is out of sync with your magenta) set the environment variable MAGENTA to the location of your magenta and build with the gccwrap.sh script:
```
MAGENTA=$HOME/magenta CGO_ENABLED=1 CC_FOR_TARGET=$(pwd)/../misc/fuchsia/gccwrap.sh GOOS=fuchsia ./make.bash
```