[Sparc] Fix double-float fabs and fneg on little endian CPUs.

The SparcV8 fneg and fabs instructions interestingly come only in a
single-float variant. Since the sign bit is always the topmost bit no
matter what size float it is, you simply operate on the high
subregister, as if it were a single float.

However, the layout of double-floats in the float registers is reversed
on little-endian CPUs, so that the high bits are in the second
subregister, rather than the first.

Thus, this expansion must check the endianness to use the correct
subregister.

llvm-svn: 267489