add wasm-simd support for suggestVectorSizeForCpu

[kounoike]

std.simd.suggestVectorSizeForCpu didn't support Wasm SIMD
This PR adds Wasm SIMD support.

$ zig version
0.11.0-dev.1987+a2c6ecd6d
$ cat wasm-simd.zig 
const std = @import("std");

pub fn main() !void {
    const wasm_simd_model = std.Target.wasm.cpu.bleeding_edge;
    const wasm_simd_cpu = comptime std.Target.Cpu.Model.toCpu(&wasm_simd_model, std.Target.Cpu.Arch.wasm32);

    std.debug.print("vector size for u8(wasm) {?}\n", .{std.simd.suggestVectorSizeForCpu(u8, wasm_simd_cpu)});
}
$ zig run wasm-simd.zig 
vector size for u8(wasm) null
$ zig run --zig-lib-dir ~/ghq/github.com/ziglang/zig/lib wasm-simd.zig 
vector size for u8(wasm) 16