[Flang] NFC: Update test to work on Mac (llvm#144253)

`%flang` expands to `flang -isysroot <SDK location>` in Mac and probably
other OS as well. `fc1` is only accepted as the first argument and hence
in this case it fails.

Use the `%flang_fc1` option to correctly expand to `flang -fc1 -isysroot
<SDK location>`.

Author: kiranchandramohan

flang/test/Preprocessing/bug518.F

@@ -1,4 +1,4 @@
-! RUN: %flang -fc1 -fdebug-unparse %s 2>&1 | FileCheck %s
+! RUN: %flang_fc1 -fdebug-unparse %s 2>&1 | FileCheck %s
 ! CHECK: k=1_4
                         k=                                            1_99999999
      &4