libffi: Updated to use 3.0.13

* Updated to use 3.0.13 version
* Fixed issue with host lib paths (on certain 64-bit hosts, namely RHEL)
* Update the MD5 and SHA256 hashes for tarball

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>
external-xilinx-toochain: Created recipe

* Allows use of external Xilinx MicroBlaze and Zynq pre-built toolchains
* Does not require additional layer dependencies
* Backported from master

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>