[x32] Add AsmBackend for X32 which uses ELF32 with x86_64 (the author is Pavel Chupin).

This is minimal change for backend required to have "hello world" compiled and working on x32 target (x86_64-linux-gnux32). More patches for x32 will follow.

Differential Revision: http://reviews.llvm.org/D4181



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212716 91177308-0d34-0410-b5e6-96231b3b80d8