Fix the dbg! macro for no_std mode.

Check for the `std` feature outside the macro body rather than inside,
so that we get the `std` feature in `cretonne-codegen`, rather than in
whatever crate the macro is being expanded in.

lib/codegen/src/dbg.rs

@@ -110,6 +110,7 @@ fn open_file() -> io::BufWriter<File> {
 /// Write a line to the debug trace file if tracing is enabled.
 ///
 /// Arguments are the same as for `printf!`.
+#[cfg(feature = "std")]
 #[macro_export]
 macro_rules! dbg {
     ($($arg:tt)+) => {
@@ -121,6 +122,13 @@ macro_rules! dbg {
     }
 }
 
+/// `dbg!` isn't supported in `no_std` mode, so expand it into nothing.
+#[cfg(not(feature = "std"))]
+#[macro_export]
+macro_rules! dbg {
+    ($($arg:tt)+) => {}
+}
+
 /// Helper for printing lists.
 pub struct DisplayList<'a, T>(pub &'a [T])
 where