esp32: implement task based scheduler

This has been a *lot* of work, trying to understand the Xtensa windowed
registers ABI. But in the end I managed to come up with a very simple
implementation that so far seems to work very well.

I tested this with both blinky examples (with blinky2 slightly edited)
and ./testdata/coroutines.go to verify that it actually works.
Most development happened on the ESP32 QEMU fork from Espressif
(https://github.com/espressif/qemu/wiki) but I also verified that it
works on a real ESP32.

src/runtime/arch_xtensa.go

@@ -2,6 +2,8 @@
 
 package runtime
 
+import "device"
+
 const GOARCH = "arm" // xtensa pretends to be arm
 
 // The bitness of the CPU (e.g. 8, 32, 64).
@@ -12,4 +14,7 @@ func align(ptr uintptr) uintptr {
 	return (ptr + 3) &^ 3
 }
 
-func getCurrentStackPointer() uintptr
+func getCurrentStackPointer() uintptr {
+	// The stack pointer (sp) is a1.
+	return device.AsmFull("mov {}, sp", nil)
+}