[WebAssembly] Add support for debug (DWARF) sections

Specifically add support for custom sections that contain
relocations, and for the two new relocation types needed
by DWARF sections.

See: https://reviews.llvm.org/D44184

Patch by Yury Delendik!

Differential Revision: https://reviews.llvm.org/D44184

llvm-svn: 331566