MC: correct DWARF header for PE/COFF assembly input

The header contains an offset to the DWARF line table for the CU.  The offset
must be section relative for COFF and absolute for others.  The non-assembly
code path for the DWARF header generation already has the correct emission for
the headers.  This corrects the assembly input path.

This was identified by BFD objecting to the LLVM generated DWARF information.