[WebAssembly] Create section start symbols automatically for all sections

These symbols only get included in the output symbols table if
they are used in a relocation.

This behaviour matches more closely the ELF object writer.

Differential Revision: https://reviews.llvm.org/D46561

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@332005 91177308-0d34-0410-b5e6-96231b3b80d8