pcg-c: Add 'static' to inline function definitions

Make the inlines static to avoid kernel build failure with Clang 11 on i386.
(The issue was not observed with Clang 10, currently in tree; reproduc

pcg-c: Add 'static' to inline function definitions

Make the inlines static to avoid kernel build failure with Clang 11 on i386.
(The issue was not observed with Clang 10, currently in tree; reproduction
depends on compiler inlining choices.)

The compiler may choose not to inline 'bare' C inlines, and in that case
expects a symbol of the same name will be available. It does not
automatically define that symbol at use, because of traditional C linking
semantics. (In contrast, C++ does define it, and then deduplicates redundant
definitions at link). As we do not instantiate the C99 inline ('extern
inline ...;'), the linker errors with "undefined symbol."

Reported by: dim
Tested by: dim
Fixes: r364219

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Add prng(9) API

Add prng(9) as a replacement for random(9) in the kernel.

There are two major differences from random(9) and random(3):

- General prng(9) APIs (prng32(9), etc) do not guarantee an

Add prng(9) API

Add prng(9) as a replacement for random(9) in the kernel.

There are two major differences from random(9) and random(3):

- General prng(9) APIs (prng32(9), etc) do not guarantee an
implementation or particular sequence; they should not be used for
repeatable simulations.

- However, specific named API families are also exposed (for now: PCG),
and those are expected to be repeatable (when so-guaranteed by the named
algorithm).

Some minor differences from random(3) and earlier random(9):

- PRNG state for the general prng(9) APIs is per-CPU; this eliminates
contention on PRNG state in SMP workloads. Each PCPU generator in an
SMP system produces a unique sequence.

- Better statistical properties than the Park-Miller ("minstd") PRNG
(longer period, uniform distribution in all bits, passes
BigCrush/PractRand analysis).

- Faster than Park-Miller ("minstd") PRNG -- no division is required to
step PCG-family PRNGs.

For now, random(9) becomes a thin shim around prng32(). Eventually I
would like to mechanically switch consumers over to the explicit API.

Reviewed by: kib, markj (previous version both)
Discussed with: markm
Differential Revision: https://reviews.freebsd.org/D25916

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Import PCG-C into sys/contrib

The intended (future) use is to provide fast pseudo-random numbers in non-
cryptographic applications.