Directly accept GOARM env var for ARM version

As GOARCH defaults to 'arm' on arm systems this allows packagers to
specify the arm version by setting the GOARM env var to 5, 6 or 7.

Packagers can then simply put something like this before the build.go call
into the build script. (The same way it is already done for [go itself](https://github.com/archlinuxarm/PKGBUILDs/blob/master/community/go/PKGBUILD#L38)).

```bash
[ "$CARCH" == 'arm' ] && export GOARM=5
[ "$CARCH" == 'armv6h' ] && export GOARM=6
[ "$CARCH" == 'armv7h' ] && export GOARM=7
```