Fix #64780 (PHP 5.5 builds are broken with GCC 3)

Fix #64780 (PHP 5.5 builds are broken with GCC 3)

A recent change (by me) introduced a call to __builtin_offsetof()
into zend_operators.h which is not defined by GCC prior to
version 4.

Changed the code to use offsetof() instead: this is defined in
<stddef.h>, so #include this header conditionally (#ifdef GNUC)