[llvm-objdump][WebAssembly] Fix llvm-objdump on files without symbols

If a file has no symbols, perhaps because it is a linked executable,
synthesize some symbols by walking the code section.  Otherwise the
disassembler will try to treat the whole code section as a function,
which won't parse.  Fixes https://bugs.llvm.org/show_bug.cgi?id=50957.

Differential Revision: https://reviews.llvm.org/D105539