Add Uart::availableForWrite()

[sandeepmistry]

As discussed in arduino/Arduino#5006.

[ArduinoBot]

✅ Build completed.

⬇️ Build URL: http://downloads.arduino.cc/PR/samd/package_samd-b80_index.json

ℹ️ To test this build:

1. Open the Preferences of the Arduino IDE.
2. Add the Build URL above in the Additional Boards Manager URLs field, and click OK.
3. Open the Boards Manager (menu Tools->Board->Board Manager...)
4. Install Arduino SAMD core - Pull Request Add Uart::availableForWrite() #146
5. Select one of the boards under SAMD Pull Request Add Uart::availableForWrite() #146 in Tools->Board menu
6. Compile/Upload as usual

[PaulStoffregen]

Looks good. I recommend merging ASAP.