[WebAssembly] MC: Refactor section creation code

Remove the use of default argument in favor of a separate
startCustomSection method.

Differential Revision: https://reviews.llvm.org/D45794

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@330632 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed