Add AVR target and toolchain to Clang

Summary:
Authored by Senthil Kumar Selvaraj

This patch adds barebones support in Clang for the (experimental) AVR target. It uses the integrated assembler for assembly, and the GNU linker for linking, as lld doesn't know about the target yet.

The DataLayout string is the same as the one in AVRTargetMachine.cpp. The alignment specs look wrong to me, as it's an 8 bit target and all types only need 8 bit alignment. Clang failed with a datalayout mismatch error when I tried to change it, so I left it that way for now.

Reviewers: rsmith, dylanmckay, cfe-commits, rengolin

Subscribers: rengolin, jroelofs, wdng

Differential Revision: https://reviews.llvm.org/D27123

llvm-svn: 291082