Remove "lto_on_osx" xfails, now that -rdynamic works on Darwin.

Note that this will require a recent version of the linker for Darwin
builds with LTO to pass these tests.
Add the saving of S2. This is needed for some of the floating point
helper functions. This can be optimized out later when the remaining
parts of the helper function work is moved into the Mips16HardFloat pass.
For now it forces us to use the 32 bit save/restore instructions instead
of the 16 bit ones.