package/moarvm: fix libffi select

moarvm's dyncall supports powerpc64 since version 2020.05 and
https://github.com/MoarVM/MoarVM/commit/d2e84186764bd3c45e7e10547f87ffc5a06f9efd

moarvm's dyncall supports mips since version 2020.02 and
https://github.com/MoarVM/MoarVM/commit/657b536cf6e861ac0bb4bf550e8aea5042a950be

dyncall does not support RISC-V as stated in https://dyncall.org/index

Fixes:
 - http://autobuild.buildroot.org/results/027b7d15e6a6111409ed17bb29e001b6c6d9e0cf



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>