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.

targets/esp32.json

@@ -2,13 +2,16 @@
 	"inherits": ["xtensa"],
 	"cpu": "esp32",
 	"build-tags": ["esp32", "esp"],
+	"scheduler": "tasks",
 	"linker": "xtensa-esp32-elf-ld",
+	"default-stack-size": 2048,
 	"cflags": [
 		"-mcpu=esp32"
 	],
 	"linkerscript": "targets/esp32.ld",
 	"extra-files": [
-		"src/device/esp/esp32.S"
+		"src/device/esp/esp32.S",
+		"src/internal/task/task_stack_esp32.S"
 	],
 	"binary-format": "esp32",
 	"flash-command": "esptool.py --chip=esp32 --port {port} write_flash 0x1000 {bin} -ff 80m -fm dout"