Fix "-arch arm64" flag for aarch64-ios-sim

Passing "-arch arm64" as a single string to clang causes an "argument
unused during compilation" warning that becomes an error if -Werror is
enabled. Fix passes it as two separate arguments (unfortunately
"-arch=arm64" does not work either).

src/lib.rs

@@ -2262,7 +2262,12 @@ impl Build {
                 format!("{}os", sdk_prefix)
             }
             ArchSpec::Simulator(arch) => {
-                cmd.args.push(arch.into());
+                if arch.starts_with('-') {
+                    cmd.args.push(arch.into());
+                } else {
+                    cmd.args.push("-arch".into());
+                    cmd.args.push(arch.into());
+                }
                 cmd.args
                     .push(format!("-m{}simulator-version-min={}", sim_prefix, min_version).into());
                 format!("{}simulator", sdk_prefix)