[XCOFF] make sure same number of paddings are added for DWARF sections

We are going to change alignment for DWARF sections. This patch is to
fix functionality issue if the alignment is not the same with
DefaultSectionAlign defined in XCOFFObjectWriter.cpp.

Currently no test for this patch as for now alignment for DWARF sections
and other sections are always the same. This patch will be tested when
patch changing DWARF section is merged in.

Reviewed By: Esme

Differential Revision: https://reviews.llvm.org/D116092