sd-journal: use a dynamic check for valgrind

sd-journal: use a dynamic check for valgrind

I left this one as a separate commit because it is more involved.
We want people to compile with valgrind support, but we don't want to
use a slow hash function unless we're actually running under valgrind.
So the compile-time check is changed to a runtime check. When compiled
with optimization, the compiler should elide the checks on the constants,
and only leave the check for RUNNING_ON_VALGRIND. It is wrapped with
_unlikely_ so that the else branch is put in the hot path.