Add support for le64.

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

Also see the associated LLVM change.

Test Plan: make check-all

Reviewers: dschuff

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5318

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217694 91177308-0d34-0410-b5e6-96231b3b80d8
Fix copy/paste for test, the triple should be le64-unknown-unknown

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217695 91177308-0d34-0410-b5e6-96231b3b80d8
No need to use PNaCl's calling convention since PNaCl now uses a separate approach for calling conventions.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217696 91177308-0d34-0410-b5e6-96231b3b80d8