x86: Fix build on 32-bit userlands

x86: Fix build on 32-bit userlands

If building for a 32-bit target userland, the toolchain defaults to
32-bit as well. While we gain -m64 automatically while building the
kernel module, we need to inject it explicitly for the hypervisor and
the inmates.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>