[AArch64] Generate literals by the little end

ARM seems to prefer that long literals be formed from their little end in
order to promote the fusion of the instrs pairs MOV/MOVK and MOVK/MOVK on
Cortex A57 and others (v.  "Cortex A57 Software Optimisation Guide", section
4.14).

Differential revision: https://reviews.llvm.org/D28697

llvm-svn: 292422