Reorganized the Sparc backend to be more modular -- each different

implementation of a Target{RegInfo, InstrInfo, Machine, etc} now has a separate
header and a separate implementation file.

This means that instead of a massive SparcInternals.h that forces a
recompilation of the whole target whenever a minor detail is changed, you should
only recompile a few files.

Note that SparcInternals.h is still around; its contents should be minimized.

llvm-svn: 10500