arm/run: allow tests to run in AArch32 mode

With this and the "mkstandalone: fix generation for arm" patch, we
can now send an arm standalone test to an aarch64 host and run it.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Message-Id: <1453474709-10679-5-git-send-email-drjones@redhat.com>
Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>