libffi: Update to version 3.5.1

libffi: Update to version 3.5.1

- Update from version 3.4.8 to 3.5.1
- Update of rootfile
- Changelog
    3.5.1
	Downgrade Autoconf requirement to version 2.68 by @kleisauke in #922
	Fix symbol versioning error.
    3.5.0
	Add FFI_VERSION_STRING and FFI_VERSION_NUMBER macros, as well
	 as ffi_get_version() and ffi_get_version_number() functions.
	Add ffi_get_default_abi() and ffi_get_closure_size() functions.
	Fix closures on powerpc64-linux when statically linking.
	Mark the PA stack as non-executable.

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>