Define FFI_GO_CLOSURES

The libffi shipped with Apple's XCode toolchain does not contain a
definition of the FFI_GO_CLOSURES macro, despite containing references
to said macro. Work around this by defining the macro, following the
model of a similar workaround in OpenJDK [1].

[1] https://github.com/openjdk/jdk17u-dev/pull/741/files

(cherry picked from commit 8b35e8ca)