[lanai] Add ELF enum value and relocations.

Add ELF enum value and relocations for Lanai backed.

General Lanai backend discussion on llvm-dev thread "[RFC] Lanai backend" (http://lists.llvm.org/pipermail/llvm-dev/2016-February/095118.html).

Differential Revision: http://reviews.llvm.org/D17008

llvm-svn: 262394