Add support for GOARM parameter when cross compiling

Go can cross compile to older ARM architectures (e.g. v5). In order to
do so one needs to use the GOARM parameter as documented in
https://github.com/golang/go/wiki/GoArm

doc/020_installation.rst

@@ -113,6 +113,8 @@ supply the target OS and platform via the command-line options like this
 
     $ go run build.go --goos freebsd --goarch 386
 
+    $ go run build.go --goos linux --goarch arm --goarm 6
+    
 The resulting binary is statically linked and does not require any
 libraries.