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

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

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

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

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

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

Signed-off-by: guoshichao <[email protected]>

include/nuttx/compiler.h

@@ -91,6 +91,12 @@
 
 #if defined(__ghs__)
 
+#  define __extension__
+
+/* the following function is not truly need in runtime, and in order to fix
+ * undefined symbol link error, we need to make the following symbols as weak
+ */
+
 #  pragma weak __gh_long_long_printf
 #  pragma weak __gh_float_printf
 #  pragma weak __gh_fputs_stdout