pw_arduino_build: Fix Windows related issues

- Launch teensy installer with admin rights.
  subprocess.open doesn't exec with admin rights on windows
  which causes the TeensyduinoInstaller.exe to fail.

- Fix include path directory separator using pathlib.

Change-Id: I3fac7915a640ef66c0c3bdb4729e69cb250d5444
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/22004
Commit-Queue: Anthony DiGirolamo <tonymd@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
diff --git a/pw_arduino_build/arduino.gni b/pw_arduino_build/arduino.gni
index 0fd4c3c..dd1e6f7 100644
--- a/pw_arduino_build/arduino.gni
+++ b/pw_arduino_build/arduino.gni
@@ -21,6 +21,10 @@
 
   # Expected args for an Arduino build:
   arduino_core_name = "teensy"
+
+  # TODO(tonymd): "teensy/avr" here should match the folders in this dir:
+  # "../third_party/arduino/cores/$arduino_core_name/hardware/*")
+  # For teensy: "teensy/avr", for adafruit-samd: "samd/1.6.2"
   arduino_package_name = "teensy/avr"
   arduino_board = "teensy40"