RTEMS | Stale xz.h and scattered include paths (#5314)

[Kinsey Moore (@opticron)]

Kinsey Moore created an issue: https://gitlab.rtems.org/rtems/rtos/rtems/-/issues/5314

Assignee: Amar Takhar

## Summary
With the migration of 3rd party source out of `cpukit/` and `bsps/`, this has left some things in a bad state (build issues fixed by !640), at least 1 stale header file that should have been migrated (xz.h), and formerly globally available headers no longer globally available.

The include paths that got scattered around the `spec/` tree should be cleaned up to a globally available point (`spec/build/cpukit/optincludes.yml` or `spec/build/bsps/optincludes.yml`) and any stale headers cleaned up.


## Steps to reproduce


### Pre-set options

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