Object, COFF: Relax aux symbols for section definitions

We, I suppose naïvely, believed the COFF specification with regard to
auxiliary symbol records which defined sections: they specified that the
symbol value should be zero.  However, dumpbin and MinGW's objdump do
not consider the symbol value as a restriction.  Relaxing this allows us
to properly dump MinGW linked executables.

llvm-svn: 219479