bootstrap: Move -Zexternal-macro-backtrace to builder

No need for this to be in `rustc.rs`

src/bootstrap/bin/rustc.rs

@@ -124,8 +124,6 @@ fn main() {
             cmd.arg("--sysroot").arg(&sysroot);
         }
 
-        cmd.arg("-Zexternal-macro-backtrace");
-
         // Link crates to the proc macro crate for the target, but use a host proc macro crate
         // to actually run the macros
         if env::var_os("RUST_DUAL_PROC_MACROS").is_some() {

src/bootstrap/builder.rs

@@ -872,6 +872,8 @@ impl<'a> Builder<'a> {
             rustflags.arg("-Zforce-unstable-if-unmarked");
         }
 
+        rustflags.arg("-Zexternal-macro-backtrace");
+
         let want_rustdoc = self.doc_tests != DocTests::No;
 
         // We synthetically interpret a stage0 compiler used to build tools as a