greenhills: add empty "__extension__" definition to fix build error

"libcxxmini/libxx_cxa_guard.cxx", line 47: error #77-D: this declaration has
          no storage class or type specifier
  __extension__ typedef int __guard __attribute__((mode(__DI__)));
  ^

"libcxxmini/libxx_cxa_guard.cxx", line 47: error #65: expected a ";"
  __extension__ typedef int __guard __attribute__((mode(__DI__)));
                ^

"libcxxmini/libxx_cxa_guard.cxx", line 64: error #20: identifier "__guard" is
          undefined
    int __cxa_guard_acquire(FAR __guard *g)
                                ^

"libcxxmini/libxx_cxa_guard.cxx", line 64: error #20: identifier "g" is
          undefined
    int __cxa_guard_acquire(FAR __guard *g)
                                         ^

"libcxxmini/libxx_cxa_guard.cxx", line 65: error #65: expected a ";"
    {
    ^

"libcxxmini/libxx_cxa_guard.cxx", line 93: warning #12-D: parsing restarts
          here after previous syntax error
  }

Signed-off-by: guoshichao <guoshichao@xiaomi.com>

include/nuttx/compiler.h

@@ -87,12 +87,20 @@
 #  define CONFIG_HAVE_CXX14 1
 #endif
 
+/* Green Hills Software definitions *****************************************/
+
+#if defined(__ghs__)
+
+#  define __extension__
+
+#endif
+
 /* GCC-specific definitions *************************************************/
 
 #ifdef __GNUC__
 
 /* Built-ins */
-#  if __GNUC__ >= 4
+#  if __GNUC__ >= 4 && !defined(__ghs__)
 #    define CONFIG_HAVE_BUILTIN_BSWAP16 1
 #    define CONFIG_HAVE_BUILTIN_BSWAP32 1
 #    define CONFIG_HAVE_BUILTIN_BSWAP64 1