Add support for _CR3 critical standby (S3) threshold.

Along with _PSV, _HOT, and _CRT, ACPI supports the _CR3 threshold
which specifies a temperature above which a system should transition
to the S3 standby state.

On FreeBSD, this is more useful than _HOT, which specifies the S4
transition threshold temperature (since FreeBSD does not generally
support the S4 state), or, in many cases, _CRT, since after
transitioning to S3 the system can cool and then be resumed.

Reviewed by: jhb, bcr (manpages)
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D35980