Makefile: build fuchsia go binaries using the sdk

This commit removes a dependency on fuchsia's tools/devshell/contrib/go,
which was removed recently[0].

Now to build go binaries for fuchsia we are not relying on the
fuchsia.git repo at all. The code just needs a copy of the fuchsia
//third_party/go repository, the fuchsia sdk and a copy of clang.

Users should download the sdk and clang beforehand (see documentation
for links), and then run (with the correct environment variables):

```
$ make TARGETOS=fuchsia TARGETARCH=amd64 \
    SOURCEDIR=$HOME/fuchsia \
    FX_SDK_PATH=${HOME}/sdk/fuchsia-sdk \
    CLANG_PATH=${HOME}/sdk/clang \
    fuchsia_go
```

After that, they will be able to build the syzkaller go binaries by
doing:

```
$ make TARGETOS=fuchsia TARGETARCH=amd64 \
    SOURCEDIR=$HOME/fuchsia \
    FX_SDK_PATH=${HOME}/sdk/fuchsia-sdk \
    CLANG_PATH=${HOME}/sdk/clang
```

This commits adds two scripts to tools/fuchsia, one for building go in
fuchsia (just calls the ./make.all script in fuchsia's go repo), and one
that replaces the devshell/contrib/go script.

Given that go is unsupported in fuchsia, this change might break at any
point.

[0]: https://fuchsia-review.googlesource.com/c/fuchsia/+/291631

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