microbit-v2: add support for S113 SoftDevice

This currently doesn't work with `tinygo flash` yet (even with
`-programmer=openocd`), you can use pyocd instead. For example, from the
Bluetooth package:

    tinygo build -o test.hex -target=microbit-v2-s113v7 ./examples/advertisement/
    pyocd flash --target=nrf52 test.hex

I intend to add support for pyocd to work around this issue, so that a simple
`tinygo flash` suffices.

targets/nrf52833-s113v7.json

@@ -0,0 +1,7 @@
+{
+	"build-tags": ["softdevice", "s113v7"],
+	"linkerscript": "targets/nrf52833-s113v7.ld",
+	"ldflags": [
+		"--defsym=__softdevice_stack=0x700"
+	]
+}