Consult SIZEOF_ZEND_LONG instead of SIZEOF_LONG

SIZEOF_ZEND_LONG more accurately indicates the size of PHP's integer values, while SIZEOF_LONG likely points to the platform itself (may not necessarily match). Preferring SIZEOF_ZEND_LONG may avoid some issues on Windows, as found while working on AppVeyor testing in PHPC-1131.

This is also the real value of PHP_INT_SIZE, which we frequently consult in tests.

This constant dates back to 28b7a03 and is available in all versions of PHP 7.