stress2: Don't run aesni.sh on non-x86 platforms

aesni.ko only exists on amd64 and i386, so on other platforms we are
just running geli.sh twice.

In fact, this test has little value anymore because aesni is included in
GENERIC, so it's always going to be used in preference to cryptosoft.

Reviewed by: pho
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D45366