Add AVR backend skeleton

This adds part of the target info code, and adds modifications to
the build scripts so that AVR is recognized a supported, experimental
backend.

It does not include any AVR-specific code, just the bare sources required
for a backend to exist.

From D14039.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252865 91177308-0d34-0410-b5e6-96231b3b80d8