[PATCH v3] libstdc++: Implement Philox Engine [PR119794]

[1nfocalypse]

Implements Philox Engine (P2075R6) and associated tests.

Additionally implements changes based on feedback from Patrick
Palka and jwakely on v2.

Regarding the question posed by Patrick on why some things are
defined inline and some out of line, things were either defined inline
either due to being concise (i.e. max()), to keep parity with other
generators (equality operator overload), or for functionality
(istream/ostream operators). I particularly attempted defining the
stream operators out of line because they are fairly obtrusive, however,
I couldn't get functionality out of them out of line. I am happy to revisit this
if needed.

Of further note, altering version.def via autogen led to a line
in excess of 80 chars. Since it was the result of autogen, I let it lie. If this
was incorrect, I can adjust it.

Otherwise, thank you for the review, and please let me know if further
alterations are needed.

Built/tested on x86_64-linux-gnu.
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