llvm-dwarfdump: Add support for some COFF relocations

DWARF in COFF utilizes several relocations.  Implement support for them
in RelocVisitor to support llvm-dwarfdump.
COFF: Don't oversize COMMON symbols when targeting BFD ld

COFF normally doesn't allow us to describe the alignment of COMMON
symbols.

It turns out that most linkers use the symbol size as a hint as to how
aligned the symbol should be.

However the BFD folks have added a .drectve command, which we
now support as of r219229, that allows us to specify the alignment
precisely.  With this in mind, stop rounding sizes up.