[WebAssembly] Match llvm change to custom section size

Summary:
The content of custome sections no longer includes the
name itself.

See: https://reviews.llvm.org/D45579

Subscribers: jfb, dschuff, jgravelle-google, aheejin, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D45580

llvm-svn: 329948