properly respect rustflags, fix x86-sim issue

[jkelleyrtp]

Fixes: #4238
Fixes: #4296

Tested:

• merging of rustflags
• using .cargo/config
• using RUSTFLAGS env var
• hotpatching enabled and hotpatching disabled

// main.rs
use dioxus::prelude::*;

fn main() {
    dioxus::launch(app);
}

fn app() -> Element {
    let mut buf = [0u8; 120];
    getrandom::fill(&mut buf).unwrap();

    rsx! {
        div {
            h1 { "Hello, Dioxus!" }
            p { "This is a simple Dioxus application." }
            pre { "Random bytes: {buf:?}" }
        }
    }
}

# .cargo/config.toml
[target.wasm32-unknown-unknown]
rustflags = ['--cfg', 'getrandom_backend="wasm_js"']