[WebAssembly] Import wasm_simd128.h from Emscripten

Summary:
As the WebAssembly SIMD proposal nears stabilization, there is desire
to use it with toolchains other than Emscripten. Moving the intrinsics
header to clang will make it available to WASI toolchains as well.

Reviewers: aheejin, sunfish

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76959