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.

targets/avr.json

@@ -1,5 +1,7 @@
 {
 	"build-tags": ["avr", "js", "wasm"],
+	"goos": "js",
+	"goarch": "wasm",
 	"compiler": "avr-gcc",
 	"linker": "avr-gcc",
 	"objcopy": "avr-objcopy",

targets/cortex-m.json

@@ -1,5 +1,7 @@
 {
 	"build-tags": ["tinygo.arm", "js", "wasm"],
+	"goos": "js",
+	"goarch": "wasm",
 	"compiler": "clang-7",
 	"gc": "marksweep",
 	"linker": "arm-none-eabi-ld",