Rollup merge of 21671 - akiss77:pr-aarch64-fastisel0, r=alexcrichton

Author: Manishearth

src/librustc_trans/back/write.rs

@@ -1012,6 +1012,9 @@ unsafe fn configure_llvm(sess: &Session) {
         if sess.time_llvm_passes() { add("-time-passes"); }
         if sess.print_llvm_passes() { add("-debug-pass=Structure"); }
 
+        // FIXME #21627 disable faulty FastISel on AArch64 (even for -O0)
+        if sess.target.target.arch.as_slice() == "aarch64" { add("-fast-isel=0"); }
+
         for arg in sess.opts.cg.llvm_args.iter() {
             add(&(*arg)[]);
         }