In case of variadic arguments in XPLINK, all arguments within the
variadic part are passed into GPRs if available or argument list even if
arguments are of double type. z/OS port of Libffi was missing handling
of such variadic arguments, causing us to pass double or complex type
parameters into FPRs. This commit adds new field in ffi_cif struct
providing number of fixed args in parameters and use it to ensure double
or complex type parameters within variadic part is set up correctly.

Signed-off-by: Rahil Shah <rahil@ca.ibm.com>