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 Charles Davis and Nico Rieck.