Suppress a warning on clang

The following warning appears without this fix:

```
parse.y:78:1: warning: unknown warning group '-Wpsabi', ignored
      [-Wunknown-warning-option]
RBIMPL_WARNING_IGNORED(-Wpsabi)
^
./include/ruby/internal/warning_push.h:103:39: note: expanded from macro
      'RBIMPL_WARNING_IGNORED'
                                      ^
./include/ruby/internal/warning_push.h:99:39: note: expanded from macro
      'RBIMPL_WARNING_PRAGMA2'
                                      ^
./include/ruby/internal/warning_push.h:98:39: note: expanded from macro
      'RBIMPL_WARNING_PRAGMA1'
                                      ^
./include/ruby/internal/warning_push.h:97:39: note: expanded from macro
      'RBIMPL_WARNING_PRAGMA0'
                                      ^
<scratch space>:49:27: note: expanded from here
 clang diagnostic ignored "-Wpsabi"
                          ^
1 warning generated.
```

parse.y

@@ -70,7 +70,7 @@ struct lex_context {
     BITFIELD(enum shareability, shareable_constant_value, 2);
 };
 
-#ifdef __GNUC__
+#if defined(__GNUC__) && !defined(__clang__)
 // Suppress "parameter passing for argument of type 'struct
 // lex_context' changed" notes.  `struct lex_context` is file scope,
 // and has no ABI compatibility issue.