[WebAssembly] Add prototype relaxed float to int trunc instructions

Add i32x4.relaxed_trunc_f32x4_s, i32x4.relaxed_trunc_f32x4_u,
i32x4.relaxed_trunc_f64x2_s_zero, i32x4.relaxed_trunc_f64x2_u_zero.

These are only exposed as builtins, and require user opt-in.

Differential Revision: https://reviews.llvm.org/D112186