[AVR] Allow AVR to be explicitly set as the default target triple

This extends the CMake cross compilation logic so that AVR can be set as
the default target triple, and thus the generic codegen tests can be
run.

This used to be possible on AVR; the CMake configuration files have
since been changed.

With this patch, 'cmake -DLLVM_DEFAULT_TARGET_TRIPLE=avr-unknown-unknown' can
be passed on the command line, making the `-mcpu` argument redundant to
'llc' and friends.

llvm-svn: 351678