MC: collapse emission of producer

Rather than use three EmitBytes, concatenate the string at compile time,
constructing a single StringRef and emitting the data in one shot.  This also
creates nicer assembly output.  NFC.
MC: fix MCAsmInfo usage for windows-itanium

Windows itanium uses the GNUCOFF assmebly format, not ELF.
MC: correct DWARF header for PE/COFF assembly input

The header contains an offset to the DWARF abbreviations 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 had the correct emission for
the headers.  This corrects just the assembly path.  Due to the invalid
relocation, processing of the debug information would halt previously on the
first assembly input as the associated abbreviations would be out of range as
they would have the location increased by image base and the section offset.

This address PR20332.