[compiler-rt] Remove mentions of AuroraUX in code comments and docs.

AuroraUX was an operating system project based on an OpenSolaris kernel
and an Ada userland.

There appear to be no public code or release available online anymore.
The project page on Wikipedia [1] was deleted in January 2012, and a news
item [2] from November 2012 has more details.

[1] https://en.wikipedia.org/w/index.php?title=Special:Log/delete&page=AuroraUX
[2] https://www.phoronix.com/scan.php?page=news_item&px=MTIyMTI