[lld][WebAssemlby] Fix for string merging of -dwarf-5 sections

We were mistakenly treating `.debug_str_offsets` as a string mergable
section when it is not (it contains integers not strings).  This is an
indication that we really should find a way to store flags for custom
sections.

Fixes: https://bugs.llvm.org/show_bug.cgi?id=48828
Fixes: https://bugs.chromium.org/p/chromium/issues/detail?id=1172217

Differential Revision: https://reviews.llvm.org/D103486