elf-hints.h: Allow rtld_paths.h to be included before it

Currently rtld_paths.h will #undef _PATH_ELF_HINTS in order to override
this to the right value if included afterwards, but the other way round
does not work as elf-hints.h tries to define an already-defined macro to
a potentially different value. Thus, guard the definition here so that
rtld_paths.h's definition continues to take precedence.

Note that, although all in-tree uses of _PATH_ELF_HINTS have included
rtld_paths.h already, pax-utils wants _PATH_ELF_HINTS from elf-hints.h
and so we cannot just drop the define. In-tree uses must just continue
to make sure that they include rtld_paths.h to get the right value for
libcompat builds as is already required.

Reviewed by: kib, brooks, jhb, imp
Differential Revision: https://reviews.freebsd.org/D40918