avr: add avr toolchain config sample

The avr-libc manual and most distributions build the AVR 8-bit gcc
toolchain with the "avr" (non-canonical) target.
Some experimentation also led to the conclusion that other (canonical)
targets are not very well supported, so we force the "avr" target for
crosstool-ng as well.

Some patches are required in avr-libc for a toolchain with gcc 5.x to
work. These patches are still not part of any avr-libc release version,
so the config sample currently forces 4.9.x to avoid requiring to clone
avr-libc trunk.

References:
    https://savannah.nongnu.org/bugs/?44574
    https://gcc.gnu.org/gcc-5/changes.html

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>

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