[WebAssembly] Add more support for weak symbols

Add weak symbol tests to MC
Add symbol flags to output of `llvm-readobj -t`.

Differential Revision: https://reviews.llvm.org/D34635

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@306330 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed