[WebAssembly] Allow function signature checking at link time

This change allows checking of function signatures but
does not yes enable it by default.  In this mode, linking
two objects that were compiled with a different signatures
for the same function will produce a link error.

New options for enabling and disabling this feature have been
added: (--check-signatures/--no-check-signatures).

Differential Revision: https://reviews.llvm.org/D40371

llvm-svn: 319396