avrgcc: bake path to avr-ar into avr-gcc-ar

gcc provides wrappers for binutils' ar, nm and ranlib
executables, which must be used instead when using link-time
optimisation. See also:
http://manpages.ubuntu.com/manpages/zesty/man1/aarch64-linux-gnu-gcc-ar-5.1.html

The upstream version of avr-gcc-ar searches in paths passed to
the configure script for the avr-ar binary that it wraps, falling
back to searching PATH instead. Thus currently avr-gcc-ar works on
Nix, but only if avrbinutils is already in the environment.

This change bakes the path to avr-ar into avr-gcc-ar, since its path
is known at compile time. It also no longer searches PATH, meaning the
user's local environment won't override this path.

Note that avr-gcc-nm and avr-gcc-ranlib are compiled from the same
source file as avr-gcc-ar, just with different compiler flags.

Testing on master (without avrbinutils in the environment):

    $ nix-build -A avrgcc
    $ result/bin/avr-gcc-ar --version
    result/bin/avr-gcc-ar: Cannot find binary 'avr-ar'

Testing on branch with this fix:

    $ nix-build -A avrgcc
    $ result/bin/avr-gcc-ar --version
    GNU ar (GNU Binutils) 2.26.20160125
    ...