ecl: fix build with libffi 3.3

The build was broken by the recent libffi update
(53a04a2d) because of this upstream
change:

https://github.com/libffi/libffi/commit/ef76205647bca77796882d31f6ab5e889f461f07

I have changed the usage of FFI_SYSV as gentoo suggests:

https://wiki.gentoo.org/wiki/Libffi_3.3_porting_notes/FFI_SYSV

I'm not entirely sure if that is the right call here, but I haven't
noticed any regressions in my testing and its definitely better than a
broken build.

Upstream: https://gitlab.com/embeddable-common-lisp/ecl/issues/302