sdbus++-gendir: handle missing nproc
sdbus++-gendir: handle missing nproc

The bitbake environment doesn't have nproc unless we add a
dependency on coreutils-native, which surprisingly isn't already
in the dependency graph.  That is a heavy lift for a single call,
so add an alternative to look at /proc/cpuinfo when nproc doesn't
exist.

Signed-off-by: Patrick Williams <cbb7353e6d953ef360baf960c122346276c6e320@stwcx.xyz>
Change-Id: I12d2b418cbee16ea544b85f9b4bc78605ee190af

openbmc/sdbusplus,openbmc/sdbusplus,openbmc/sdbusplus,openbmc/sdbusplus