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_cortexm.go

@@ -0,0 +1,46 @@
+// +build cortexm
+
+package runtime
+
+import (
+	"unsafe"
+
+	"device/arm"
+)
+
+const GOARCH = "arm"
+
+// The bitness of the CPU (e.g. 8, 32, 64).
+const TargetBits = 32
+
+//go:extern _heap_start
+var heapStartSymbol unsafe.Pointer
+
+//go:extern _heap_end
+var heapEndSymbol unsafe.Pointer
+
+//go:extern _globals_start
+var globalsStartSymbol unsafe.Pointer
+
+//go:extern _globals_end
+var globalsEndSymbol unsafe.Pointer
+
+//go:extern _stack_top
+var stackTopSymbol unsafe.Pointer
+
+var (
+	heapStart    = uintptr(unsafe.Pointer(&heapStartSymbol))
+	heapEnd      = uintptr(unsafe.Pointer(&heapEndSymbol))
+	globalsStart = uintptr(unsafe.Pointer(&globalsStartSymbol))
+	globalsEnd   = uintptr(unsafe.Pointer(&globalsEndSymbol))
+	stackTop     = uintptr(unsafe.Pointer(&stackTopSymbol))
+)
+
+// Align on word boundary.
+func align(ptr uintptr) uintptr {
+	return (ptr + 3) &^ 3
+}
+
+func getCurrentStackPointer() uintptr {
+	return arm.ReadRegister("sp")
+}