Fix unaligned reads/writes in X86JIT and RuntimeDyldELF.

Summary:
Introduce support::ulittleX_t::ref type to Support/Endian.h and use it in x86 JIT
to enforce correct endianness and fix unaligned accesses.

Test Plan: regression test suite

Reviewers: lhames

Subscribers: ributzka, llvm-commits

Differential Revision: http://reviews.llvm.org/D5011