Add rustflags support to config files

`build.rustflags` is treated exactly like `RUSTFLAGS`.

It is a list, so argument lists with spaces work.

`RUSTFLAGS` takes precedent, then `build.rustflags`.

src/cargo/ops/cargo_compile.rs

@@ -469,7 +469,7 @@ fn scrape_target_config(config: &Config, triple: &str)
        None => return Ok(ret),
    };
    for (lib_name, _) in table.into_iter() {
        if lib_name == "ar" || lib_name == "linker" {
        if lib_name == "ar" || lib_name == "linker" || lib_name == "rustflags" {
            continue
        }