[WebAssebmly] Remove reloc ordering constraint

The MC layer doesn't currently emit relocations in offset
order for the entire code section so this check was causing
failures on the wasm waterfall.

Perhaps we can re-instate this check if we divide the relocations
per-function, or add extra ordering the MC object writer.

Differential Revision: https://reviews.llvm.org/D44136

llvm-svn: 326765