updated README.md download links with linuxarm32 and linuxarm64 links

[rsora]

This PR follows #304

Readme links updated to include linuxarm64 binaries links and renaming linuxarm to linuxarm32 to have a clear distinction between the two architectures

Updated also instructions as the binaries in the archive are, from now on, always named arduino-cli

[rsora]

@ArduinoBot build this please

[ArduinoBot]

✅ Build completed.

⬇️ Linux 64: https://downloads.arduino.cc/PR/arduino-cli/arduino-cli-152-PR307-linux64.tar.bz2

⬇️ Linux 32: https://downloads.arduino.cc/PR/arduino-cli/arduino-cli-152-PR307-linux32.tar.bz2

⬇️ Linux ARM 32: https://downloads.arduino.cc/PR/arduino-cli/arduino-cli-152-PR307-linuxarm32.tar.bz2

⬇️ Linux ARM 64: https://downloads.arduino.cc/PR/arduino-cli/arduino-cli-152-PR307-linuxarm64.tar.bz2

⬇️ Windows: https://downloads.arduino.cc/PR/arduino-cli/arduino-cli-152-PR307-windows.zip

⬇️ OSX: https://downloads.arduino.cc/PR/arduino-cli/arduino-cli-152-PR307-macosx.zip

ℹ️ To test this build:

1. Just replace arduino-cli binary with the provided one

[gvarisco]

LGTM