avr-libc build with LLVM/Clang

[Andreas Burnickl]

Hello,

LLVM is becoming more popular and AVR is not in an experimental state
anymore. So I decided to build the avr-libc with LLVM. GCC may lose the AVR
backend in the future. So I think LLVM would be a good choice for the
future.

To build the avr-libc only small changes were needed. Like AS compiler
flags and some compiler checks. The biggest change concerns the PROGMEM
concept. My changes don't break existing code.

Is someone interested in this topic?

The following link contains a successful build with LLVM/Clang version 11
and all changes:
https://drive.google.com/file/d/1kdleWpsmwbSZyXUQ0tp-DHu0jKABRLuQ/view?usp=sharing

Br Andreas