[WebAssembly] MC: Ensure that FUNCTION_OFFSET relocations are always against function symbols.

The getAtom() method wasn't doing what we needed in all cases. We want
the symbols for the function which defines that section. We can compute
this easily enough and we know that we have at most one function in each
section.

Once this lands I will revert rL331412 which is no longer needed.

Fixes PR37409

Differential Revision: https://reviews.llvm.org/D46970

llvm-svn: 332517