gnu: Add AVR-Libc.

* gnu/packages/avr.scm: New file.
* gnu-system.am (GNU_SYSTEM_MODULES): Add avr.scm.
* gnu/packages/cross-base.scm (xgcc-avr): New variable.
* gnu/packages/bootstrap.scm (glibc-dynamic-linker): Add case for
  "avr".

Signed-off-by: Ludovic Courtès <ludo@gnu.org>