lib/libc/gen: use Lemire's algorithm for arc4random_uniform().

Daniel Lemire has published a more efficient range reduction algorithm
for finding a random number in a given range without bias, reduc

lib/libc/gen: use Lemire's algorithm for arc4random_uniform().

Daniel Lemire has published a more efficient range reduction algorithm
for finding a random number in a given range without bias, reducing the
number of divisions to none in the common case and 1 in case the initial
sample is rejected.

This speeds up performance by 22% on amd64, 15% on i386, and 70% on armv7.

os: FreeBSD
arch: amd64
cpu: Intel(R) Core(TM) i7-4910MQ CPU @ 2.90GHz
│ benchmark.out │
│ sec/op │
Arc4random_uniform 56.53n ± 0%
Fast_uniform 44.00n ± 0%
geomean 49.87n

Reviewed by: cem
Approved by: emaste
Differential Revision: https://reviews.freebsd.org/D47659

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Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

Sync with OpenBSD:

arc4random.c: In the incredibly unbelievable circumstance where
_rs_init() fails to allocate pages, don't call abort() because of
corefile data leakage concerns, but simply _exit(

Sync with OpenBSD:

arc4random.c: In the incredibly unbelievable circumstance where
_rs_init() fails to allocate pages, don't call abort() because of
corefile data leakage concerns, but simply _exit(). The reasoning
is _rs_init() will only fail if someone finds a way to apply
specific pressure against this failure point, for the purpose of
leaking information into a core which they can read. We don't
need a corefile in this instance to debug that. So take this
"lever" away from whoever in the future wants to do that.

arc4random.3: reference random(4)

arc4random_uniform.c: include stdint.h over sys/types.h

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Revert r346410 and r346411

libkern in .PATH has too many filename conflicts with libc and my -DNO_CLEAN
tinderbox didn't catch that ahead of time. Mea culpa.

Merge ^/head r337646 through r338014.

Split arc4random_uniform into it's own file and sync with OpenBSD.

PR: 182610
Obtained from: OpenBSD
MFC after: 2 weeks