math/rand: use fastrand64 if possible

math/rand: use fastrand64 if possible

Now that the top-level math/rand functions are auto-seeded by default
(issue #54880), use the runtime fastrand64 function when 1) Seed
has not been called; 2) the GODEBUG randautoseed=0 is not used.

The benchmarks run quickly and are relatively noisy, but they show
significant improvements for parallel calls to the top-level functions.

goos: linux
goarch: amd64
pkg: math/rand
cpu: 11th Gen Intel(R) Core(TM) i7-11850H @ 2.50GHz
                           │  /tmp/foo.1   │              /tmp/foo.2               │
                           │    sec/op     │    sec/op      vs base                │
Int63Threadsafe-16            11.605n ± 1%    3.094n ±  1%  -73.34% (p=0.000 n=10)
Int63ThreadsafeParallel-16   67.8350n ± 2%   0.4000n ±  1%  -99.41% (p=0.000 n=10)
Int63Unthreadsafe-16           1.947n ± 3%    1.924n ±  2%        ~ (p=0.189 n=10)
Intn1000-16                    4.295n ± 2%    4.287n ±  3%        ~ (p=0.517 n=10)
Int63n1000-16                  4.379n ± 0%    4.192n ±  2%   -4.27% (p=0.000 n=10)
Int31n1000-16                  3.641n ± 3%    3.506n ±  0%   -3.69% (p=0.000 n=10)
Float32-16                     3.330n ± 7%    3.250n ±  2%   -2.40% (p=0.017 n=10)
Float64-16                     2.194n ± 6%    2.056n ±  4%   -6.31% (p=0.004 n=10)
Perm3-16                       43.39n ± 9%    38.28n ± 12%  -11.77% (p=0.015 n=10)
Perm30-16                      324.4n ± 6%    315.9n ± 19%        ~ (p=0.315 n=10)
Perm30ViaShuffle-16            175.4n ± 1%    143.6n ±  2%  -18.15% (p=0.000 n=10)
ShuffleOverhead-16             223.4n ± 2%    215.8n ±  1%   -3.38% (p=0.000 n=10)
Read3-16                       5.428n ± 3%    5.406n ±  2%        ~ (p=0.780 n=10)
Read64-16                      41.55n ± 5%    40.14n ±  3%   -3.38% (p=0.000 n=10)
Read1000-16                    622.9n ± 4%    594.9n ±  2%   -4.50% (p=0.000 n=10)
Concurrent-16                136.300n ± 2%    4.647n ± 26%  -96.59% (p=0.000 n=10)
geomean                        23.40n         12.15n        -48.08%

Fixes #49892

Change-Id: Iba75b326145512ab0b7ece233b98ac3d4e1fb504
Reviewed-on: https://go-review.googlesource.com/c/go/+/465037
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>