package/qemu: add symlink to target qemu-system

Scripts which use the buildroot host system to execute a qemu for the compiled
buildroot output can use the symlink at host/bin/qemu-system to execute the
appropriate qemu-system for the target, for example qemu-system-riscv64.

Signed-off-by: Christian Stewart <christian@paral.in>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>