process-util: use raw_getpid() in getpid_cache() internally (#8115)

We have the raw_getpid() definition in place anyway, and it's certainly
beneficial to expose the same semantics on pre glibc 2.24 and after it
too, hence always bypass glibc for this, and always cache things on our
side.

Fixes: #8113
(cherry picked from commit 996def17)