[WebAssembly] Add lowering for llvm.rint and llvm.roundeven

[WebAssembly] Add lowering for llvm.rint and llvm.roundeven

WebAssembly doesn't expose inexact exceptions, so frint can be mapped to
fnearbyint. Likewise, WebAssembly always rounds ties-to-even, so
froundeven can be mapped to fnearbyint.

Differential Revision: https://reviews.llvm.org/D153451