[SystemZ] Fix FPR dwarf numbering

The dwarf FPR numbers are supposed to have the order F0, F2, F4, F6,
F1, F3, F5, F7, F8, etc., which matches the pairing of registers for
long doubles.  E.g. a long double stored in F0 is paired with F2.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212701 91177308-0d34-0410-b5e6-96231b3b80d8