[WebAssembly] Add intrinsic for i64x2.mul

Summary:
This instruction was implemented in 3181273b, but that commit did
not add an intrinsic for it.

Reviewers: aheejin

Subscribers: dschuff, sbc100, jgravelle-google, sunfish, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81757