[WebAssembly] Improve wasm test cases

Add test for weakly defined symbols with the same name

Improve test for call-indirect to include the same call in two
different objects. This lays the ground work to improve the
output via de-duplicating the indirect call table:
   https://reviews.llvm.org/D40989

Also make all tests consistently pass -mtriple rather than
declaring in the sources.

Differential Revision: https://reviews.llvm.org/D41024

llvm-svn: 320172