Fix ZEND_FASTCALL definition wrt. x64 Windows clang builds (GH-15425)

As is, MSVC uses `__vectorcall`, but clang uses `__cdecl`.  This
obviously is bad for interoperability (and causes link issues), and is
likely worse for FFI which offers some limited (but likely sufficient
for our purposes) support for `__vectorcall` on Windows.

Since clang claims to support `__vectorcall` as of 3.6.0, and we bumped
the requirements to clang 4.0.0 already, there shouldn't be any issues.

Author: cmb69

Zend/zend_portability.h

@@ -306,7 +306,7 @@ char *alloca();
 # define ZEND_FASTCALL __attribute__((fastcall))
 #elif defined(_MSC_VER) && defined(_M_IX86) && _MSC_VER == 1700
 # define ZEND_FASTCALL __fastcall
-#elif defined(_MSC_VER) && _MSC_VER >= 1800 && !defined(__clang__)
+#elif defined(_MSC_VER) && _MSC_VER >= 1800
 # define ZEND_FASTCALL __vectorcall
 #else
 # define ZEND_FASTCALL