Add support for le64.

Summary:
le64 is a generic little-endian 64-bit processor, mimicking le32.

Depends on D5318.

Test Plan: make check-all

Reviewers: dschuff

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5319

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217697 91177308-0d34-0410-b5e6-96231b3b80d8
FileCheckize. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217698 91177308-0d34-0410-b5e6-96231b3b80d8