[PowerPC64LE] Fix gdtoa configurations on LE.

gdtoa wins the award for "most outdated endianness naming convention"
with its IEEE_8087 vs IEEE_MC68k defines. I had a good chuckle.

Update softfloat and arith.h to adjust to BE or LE automatically
based on the low level preprocessor defines.

Fixes printf/scanf on PowerPC64LE, although there is still a problem
lurking regarding Signalling NaNs...

Sponsored by: Tag1 Consulting, Inc.