Fix wrong code offset for unwind code SET_FPREG.

The code offset for unwind code SET_FPREG is wrong because it is set
to constant 0. The fix is to do the same as for the other unwind
codes: emit a label and later the absolute difference between the
label and the begin of the prologue.
Also enables the failing test case MC/COFF/seh.s

Reviewed by Jim Grosbach, Charles Davis and Nico Rieck.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189309 91177308-0d34-0410-b5e6-96231b3b80d8