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.

src/runtime/arch_arm.go

@@ -1,3 +1,5 @@
+// +build arm,!avr,!cortexm
+
 package runtime
 
 const GOARCH = "arm"

src/runtime/arch_cortexm.go

@@ -1,4 +1,4 @@
-// +build tinygo.arm
+// +build cortexm
 
 package runtime
 

src/runtime/arch_wasm.go

@@ -1,4 +1,4 @@
-// +build wasm,!tinygo.arm,!avr
+// +build wasm
 
 package runtime
 

src/runtime/override_js.go

@@ -1,28 +0,0 @@
-// +build js,tinygo.arm avr
-
-package runtime
-
-// This file stubs out some external functions declared by the syscall/js
-// package. They cannot be used on microcontrollers.
-
-type js_ref uint64
-
-//go:linkname js_valueGet syscall/js.valueGet
-func js_valueGet(v js_ref, p string) js_ref {
-	return 0
-}
-
-//go:linkname js_valueNew syscall/js.valueNew
-func js_valueNew(v js_ref, args []js_ref) (js_ref, bool) {
-	return 0, true
-}
-
-//go:linkname js_valueCall syscall/js.valueCall
-func js_valueCall(v js_ref, m string, args []js_ref) (js_ref, bool) {
-	return 0, true
-}
-
-//go:linkname js_stringVal syscall/js.stringVal
-func js_stringVal(x string) js_ref {
-	return 0
-}

src/runtime/runtime_cortexm.go

@@ -1,4 +1,4 @@
-// +build tinygo.arm
+// +build cortexm
 
 package runtime
 

src/runtime/runtime_unix.go

@@ -1,4 +1,4 @@
-// +build darwin linux
+// +build darwin linux,!avr,!cortexm
 
 package runtime
 

src/runtime/runtime_wasm.go

@@ -1,4 +1,4 @@
-// +build wasm,!tinygo.arm,!avr
+// +build wasm
 
 package runtime