Description:
------------
On a 32-bit system compile a php embedded program with clang will result warning or error like this:

In file included from a.c:1:
In file included from /usr/include/php/20160303/main/php.h:36:
In file included from /usr/include/php/20160303/Zend/zend.h:36:
/usr/include/php/20160303/Zend/zend_hash.h:141:15: error: fastcall calling convention ignored on
      variadic function [-Werror,-Wignored-attributes]
ZEND_API void ZEND_FASTCALL 2(HashTable *ht, apply_func_args_t app...
              ^
/usr/include/php/20160303/Zend/zend_portability.h:239:39: note: expanded from macro 'ZEND_FASTCALL'
# define ZEND_FASTCALL __attribute__((fastcall))
                                      ^
1 error generated.

This causes configuring some programs embed php failing.  It would be great if we can make ZEND_FASTCALL on this function conditional by compiler type.

Test script:
---------------
/* compile with clang on a 32-bit system */
#include <php.h>
#include <php_main.h>

int main() {
        php_request_startup();
        return 0;
}