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/runtime_cortexm.go

@@ -0,0 +1,55 @@
+// +build cortexm
+
+package runtime
+
+import (
+	"device/arm"
+	"unsafe"
+)
+
+//go:extern _sbss
+var _sbss unsafe.Pointer
+
+//go:extern _ebss
+var _ebss unsafe.Pointer
+
+//go:extern _sdata
+var _sdata unsafe.Pointer
+
+//go:extern _sidata
+var _sidata unsafe.Pointer
+
+//go:extern _edata
+var _edata unsafe.Pointer
+
+func preinit() {
+	// Initialize .bss: zero-initialized global variables.
+	ptr := uintptr(unsafe.Pointer(&_sbss))
+	for ptr != uintptr(unsafe.Pointer(&_ebss)) {
+		*(*uint32)(unsafe.Pointer(ptr)) = 0
+		ptr += 4
+	}
+
+	// Initialize .data: global variables initialized from flash.
+	src := uintptr(unsafe.Pointer(&_sidata))
+	dst := uintptr(unsafe.Pointer(&_sdata))
+	for dst != uintptr(unsafe.Pointer(&_edata)) {
+		*(*uint32)(unsafe.Pointer(dst)) = *(*uint32)(unsafe.Pointer(src))
+		dst += 4
+		src += 4
+	}
+}
+
+func abort() {
+	for {
+		arm.Asm("wfi")
+	}
+}
+
+// Implement memset for compiler-rt.
+//go:export memset
+func memset(ptr unsafe.Pointer, c byte, size uintptr) {
+	for i := uintptr(0); i < size; i++ {
+		*(*byte)(unsafe.Pointer(uintptr(ptr) + i)) = c
+	}
+}