tests: simple_ruby_test's valid_host is X86 only

tests: simple_ruby_test's valid_host is X86 only

This is because the binary is dynamically linked thus it can run
on a x86 host only.

Change-Id: I7391414fdcd8f861f62e54c4d681e29360eb7443
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26103
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>