all: add support for more architectures and GOOS/GOARCH (#118)

This commit does two things:

  * It adds support for the GOOS and GOARCH environment variables. They
    fall back to runtime.GO* only when not available.
  * It adds support for 3 new architectures: 386, arm, and arm64. For
    now, this is Linux-only.

main.go

@@ -52,6 +52,8 @@ func Compile(pkgName, outpath string, spec *TargetSpec, config *BuildConfig, act
 	compilerConfig := compiler.Config{
 		Triple:     spec.Triple,
 		CPU:        spec.CPU,
+		GOOS:       spec.GOOS,
+		GOARCH:     spec.GOARCH,
 		GC:         config.gc,
 		CFlags:     spec.CFlags,
 		LDFlags:    spec.LDFlags,