fixed point multiplication on wasm32

[archaistvolts]

This issue tracks support for fixed point multiplication on WebAssembly.

Currently, trying to build on wasm produces the following error:

// saturation.zig
test "mulWithSaturation" {
    var a: i8 = 1;
    var b: i8 = 2;
    var c = @mulWithSaturation(a, b);
    try std.testing.expect(c == 3);
}

$ zig test saturation.zig -target wasm32-wasi
LLVM ERROR: Unable to expand fixed point multiplication.
Aborted

long term: we'll have a shim that we use on backends that don't support it with machine instructions, and use it for webassembly if the llvm backend does not support it

Related #1284

[archaistvolts]

TODO: once we have a shim in place, remove this line which skips the test on wasm: https://github.com/ziglang/zig/pull/9619/files#diff-670d5a3d27d8213e735d66b4d468abe1ad52800273f84a76a611473559ac7fccR83