Properly build fallback code

build.rs

@@ -23,7 +23,8 @@ fn main() {
         cfg.file(if msvc {"src/arch/i686.asm"} else {"src/arch/i686.S"});
         cfg.define("X86", None);
     } else {
-        cfg.file("fallback.rs");
+        println!("cargo:rustc-cfg=fallback");
+        return;
     }
 
     cfg.include("src/arch").compile("libstacker.a");

src/arch/fallback.rs

@@ -10,4 +10,7 @@ unsafe extern fn __stacker_switch_stacks(
     _dataptr: &mut &mut FnMut(),
 ) {
     panic!("not supported")
-}
+}
+
+#[no_mangle]
+extern fn __stacker_black_box(_: *const u8) {}

src/lib.rs

@@ -35,6 +35,10 @@ extern crate libc;
 
 use std::cell::Cell;
 
+#[cfg(fallback)]
+#[path="arch/fallback.rs"]
+mod fallback;
+
 extern {
     fn __stacker_stack_pointer() -> usize;
     fn __stacker_switch_stacks(new_stack: usize,