all: pretend to be linux/arm in baremetal targets

So far, we've pretended to be js/wasm in baremetal targets to make the
stdlib happy. Unfortunately, this has various problems because
syscall/js (a dependency of many stdlib packages) thinks it can do JS
calls, and emulating them gets quite hard with all changes to the
syscall/js packages in Go 1.12.

This commit does a few things:
  * It lets baremetal targets pretend to be linux/arm instead of
    js/wasm.
  * It lets the loader only select particular packages from the src
    overlay, instead of inserting them just before GOROOT. This makes it
    possible to pick which packages to overlay for a given target.
  * It adds a baremetal-only syscall package that stubs out almost all
    syscalls.

targets/avr.json

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

targets/cortex-m.json

@@ -1,7 +1,7 @@
 {
-	"build-tags": ["tinygo.arm", "js", "wasm"],
-	"goos": "js",
-	"goarch": "wasm",
+	"build-tags": ["cortexm", "linux", "arm"],
+	"goos": "linux",
+	"goarch": "arm",
 	"compiler": "clang-8",
 	"gc": "marksweep",
 	"linker": "arm-none-eabi-ld",