systemd: Use cross cpp

While building for non x86 arch (ppc) found that build
was using host's cpp which was not generating exact
syscall list for ppc since it was using the include files
from host

With this patch will use $(CPP) instead of hardcoding
cpp in makefile.am and then set CPP properly in recipe

Fixes build failures on cross builds

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>