Next PPC long double bits. First cut at constants.

Next PPC long double bits.  First cut at constants.
No compile-time support for constant operations yet,
just format transformations.  Make readers and
writers work.  Split constants into 2 doubles in
Legalize.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@42865 91177308-0d34-0410-b5e6-96231b3b80d8