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
Fix copy/paste for test, the triple should be le64-unknown-unknown
No need to use PNaCl's calling convention since PNaCl now uses a separate approach for calling conventions.