sys/_pv_entry.h: Include sys/param.h

sys/param.h is required for this file because it uses howmany() which is
defined there. For the kernel, this works today because of namespace
polllution. However, user land programs, like qemu, can include
machine/pmap.h without having included sys/param.h (since it wasn't
required before).

Sponsored by: Netflix
Reviewed by: tsoome, jhb
Differential Revision: https://reviews.freebsd.org/D36927

<sys/_pv_entry>: Add a fallback for __LONG_WIDTH__ for older compilers.

clang only includes a __LONG_WIDTH__ builtin in clang 14 and later.

sys/_pv_entry.h: Drop UCB copyright as this is all Peter's code.

Reported by: alc

sys: Consolidate common implementation details of PV entries.

Add a <sys/_pv_entry.h> intended for use in <machine/pmap.h> to
define struct pv_entry, pv_chunk, and related macros and inline
functions.

Note that powerpc does not yet use this as while the mmu_radix pmap
in powerpc uses the new scheme (albeit with fewer PV entries in a
chunk than normal due to an used pv_pmap field in struct pv_entry),
the Book-E pmaps for powerpc use the older style PV entries without
chunks (and thus require the pv_pmap field).

Suggested by: kib
Reviewed by: kib
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D36685