mips: fix endian issues for LWL, LWR, LDR and LDL for mips64

This change:
- fixes endian issues for unaligned loads for MIPS64,
- (re)moves endian dependencies in guest-to-IR for Iop_ReinterpI32asF32
   and Iop_ReinterpI64asF64 to host-mips-isel,
- adds minor style changes in the area touched by the code.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2709 8f6e269a-dfd6-0310-a8e1-e2731360e62c