[PATCH v6] libstdc++: Implement Philox Engine (PR119794)

[1nfocalypse]

Implements Philox Engine (P2075R6) and associated tests.

Implements additional feedback from v5 from Patrick Palka.
Also cut some trailing whitespace from the limb propagation
fix in v5.

Apologies for the delay from v5 - I had to finish writing some
CTF challenges for DEFCON this week. Thank you for the
feedback, and please let me know if anything further needs
to be changed.

Built/tested on x86_64-pc-linux-gnu.
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