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/device/esp/esp32.S

@@ -24,7 +24,7 @@ call_start_cpu0:
     wsr.ps a2
     rsync
 
-    // Set WINDOWBASE to 1 << WINDOWSTART.
+    // Set WINDOWSTART to 1 << WINDOWBASE.
     rsr.windowbase  a2
     ssl  a2
     movi a2, 1
@@ -43,7 +43,7 @@ call_start_cpu0:
     rsync
 
     // Jump to the runtime start function written in Go.
-    j main
+    call4 main
 
 .section .text.tinygo_scanCurrentStack
 .global tinygo_scanCurrentStack