Macro wasmer_compiler_singlepass::emitter_x64::dynasm

macro_rules! dynasm {
    ($a:expr ; $($tt:tt)*) => { ... };
}

Force dynasm! to use the correct arch (x64) when cross-compiling. dynasm! proc-macro tries to auto-detect it by default by looking at the target_arch, but it sees the target_arch of the proc-macro itself, which is always equal to host, even when cross-compiling.