compiler,runtime: implement stack-based scheduler

This scheduler is intended to live along the (stackless) coroutine based
scheduler which is needed for WebAssembly and unsupported platforms. The
stack based scheduler is somewhat simpler in implementation as it does
not require full program transform passes and supports things like
function pointers and interface methods out of the box with no changes.

Code size is reduced in most cases, even in the case where no scheduler
scheduler is used at all. I'm not exactly sure why but these changes
likely allowed some further optimizations somewhere. Even RAM is
slightly reduced, perhaps some global was elminated in the process as
well.

src/runtime/runtime_cortexm.go

@@ -40,6 +40,29 @@ func preinit() {
 	}
 }
 
+// calleeSavedRegs is the list of registers that must be saved and restored when
+// switching between tasks. Also see scheduler_cortexm.S that relies on the
+// exact layout of this struct.
+type calleeSavedRegs struct {
+	r4  uintptr
+	r5  uintptr
+	r6  uintptr
+	r7  uintptr
+	r8  uintptr
+	r9  uintptr
+	r10 uintptr
+	r11 uintptr
+}
+
+// prepareStartTask stores fn and args in some callee-saved registers that can
+// then be used by the startTask function (implemented in assembly) to set up
+// the initial stack pointer and initial argument with the pointer to the object
+// with the goroutine start arguments.
+func (r *calleeSavedRegs) prepareStartTask(fn, args uintptr) {
+	r.r4 = fn
+	r.r5 = args
+}
+
 func abort() {
 	// disable all interrupts
 	arm.DisableInterrupts()