systemd.bbclass: Fixes for living along with multilib

This essentially fixes two problems
1. We were ignoring systemd for multilibs now we dont
2. We were replacing '-systemd' string in PN which is
a problem whenre -systemd appears more than once
e.g. lib32-systemd-units-system it was removing both
essentially we should only chop the end of PN if it
ends with '-systemd'

Signed-off-by: Khem Raj <raj.khem@gmail.com>