libffi: fix native build being not portable

Without `--with-gcc-arch=generic` libffi looks up the cpuid to automatically determine
which mtune/march to use. This makes the native sstate-cache unuseable since it's
possible to have a newer cpu building the recipe and the library being pulled from
another older cpu which doesn't have the instruction.

Signed-off-by: Maxime Roussin-Bélanger <maxime.roussinbelanger@gmail.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>