Support building x86 and arm64 without FFI_GO_CLOSURES (#586) * x86: Support building without FFI_GO_CLOSURES Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com> * arm: Support building without FFI_GO_CLOSURES Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>