[WebAssembly] Fix casting MCSymbol to MCSymbolWasm on ELF

Summary:
wasm32-unknown-unknown-elf has MCSymbols that are not MCSymbolWasms, so
we need a non-asserting cast here.

Reviewers: dschuff, sunfish

Subscribers: jfb, sbc100, aheejin, llvm-commits

Differential Revision: https://reviews.llvm.org/D43205

llvm-svn: 324942