linux: implement sched_getattr()

[ngie]

Summary

This change adds the necessary glue to translate Linux scheduler policy
and priority from FreeBSD so this information is available to Linux
applications.

This unbreaks using the Linux version of Chromium on FreeBSD.

This change was originally submitted via [freebsd/freebsd-src#2370][github-pr].

PR: 297468
Co-Authored-By: Enji Cooper <[e-mail]>
Signed-off-by: Joao Bonifacio <[e-mail]>

[github-pr]: https://github.com/freebsd/freebsd-src/pull/2370

Test Plan

TODO:

• build change and test as shown in the PR (minimum bar)
• run the relevant sched_getattr tests from LTP.