Fix acpica macros that subtract null pointers

[dim]

Summary

Clang 13.0.0 produces a new -Werror warning about the ACPI_TO_INTEGER(p)
and ACPI_OFFSET(d, f) macros in acpica's actypes.h:

sys/contrib/dev/acpica/components/dispatcher/dsopcode.c:708:31: error: performing pointer subtraction with a null pointer has undefined behavior [-Werror,-Wnull-pointer-subtraction]
    ObjDesc->Region.Address = ACPI_PTR_TO_PHYSADDR (Table);
                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
sys/contrib/dev/acpica/include/actypes.h:664:41: note: expanded from macro 'ACPI_PTR_TO_PHYSADDR'
#define ACPI_PTR_TO_PHYSADDR(i)         ACPI_TO_INTEGER(i)
                                        ^~~~~~~~~~~~~~~~~~
sys/contrib/dev/acpica/include/actypes.h:661:41: note: expanded from macro 'ACPI_TO_INTEGER'
#define ACPI_TO_INTEGER(p)              ACPI_PTR_DIFF (p, (void *) 0)
                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
sys/contrib/dev/acpica/include/actypes.h:656:82: note: expanded from macro 'ACPI_PTR_DIFF'
#define ACPI_PTR_DIFF(a, b)             ((ACPI_SIZE) (ACPI_CAST_PTR (UINT8, (a)) - ACPI_CAST_PTR (UINT8, (b))))
                                                                                 ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

This problem of undefined behavior was also reported to acpica by @cem
in 2018: https://github.com/acpica/acpica/issues/407, but it seems there
was never any fix committed for it upstream.

Instead fix these locally, for ACPI_TO_INTEGER by simply casting the
incoming pointer to ACPI_SIZE (which corresponds roughly to uintptr_t
and size_t), and for ACPI_OFFSET by reusing our offsetof definition from
sys/cdefs.h.

I didn't want to use our offsetof() or builtin_offsetof(), as this
might not be acceptable for upstream. But if upstream is not going to
accept this patch anyway, we might as well just use or own macros.

[kib]

Until something is fixed upstream, it might be better to use #ifdef __FreeBSD block, and then using our sys/cdefs.h facilities for FreeBSD would be both safer and better explain the intent.

Just my PoV, the last call is the ACPI maintainer' word of course.

[imp]

I'm good with this or kib's suggestion

[dim]

Use conditional FreeBSD block, and our own __offsetof().

[emaste]

LGTM, and it's not the end of the world for us to revert if upstream addresses it differently. But let's give @jkim a couple of days.