Support parallel testing in libgomp: fallback Perl 'flock' [PR66005]

Follow-up to commit 6c3b30ef9e0578509bdaf59c13da4a212fe6c2ba
"Support parallel testing in libgomp, part II [PR66005]"
("..., and enable if 'flock' is available for serializing execution testing"),
where we saw:

> On my Dell Precision 7530 laptop:
>
>     $ uname -srvi
>     Linux 5.15.0-71-generic #78-Ubuntu SMP Tue Apr 18 09:00:29 UTC 2023 x86_64
>     $ grep '^model name' < /proc/cpuinfo | uniq -c
>          12 model name      : Intel(R) Core(TM) i7-8850H CPU @ 2.60GHz
>     $ nvidia-smi -L
>     GPU 0: Quadro P1000 (UUID: GPU-e043973b-b52a-d02b-c066-a8fdbf64e8ea)
>
> ... [...]: case (c) standard configuration, no offloading
> configured, [...]

>     $ \time make check-target-libgomp
>
> Case (c), baseline; [...]:
>
>     1180.98user 110.80system 19:36.40elapsed 109%CPU (0avgtext+0avgdata 505148maxresident)k
>     1133.22user 111.08system 19:35.75elapsed 105%CPU (0avgtext+0avgdata 505212maxresident)k
>
> Case (c), parallelized [using 'flock']:
>
> [...]
>     -j12 GCC_TEST_PARALLEL_SLOTS=12
>     2591.04user 192.64system 4:44.98elapsed 976%CPU (0avgtext+0avgdata 505216maxresident)k
>     2581.23user 195.21system 4:47.51elapsed 965%CPU (0avgtext+0avgdata 505212maxresident)k

Quite the same when instead of 'flock' using this fallback Perl 'flock':

    2565.23user 194.35system 4:46.77elapsed 962%CPU (0avgtext+0avgdata 505216maxresident)k
    2549.38user 200.20system 4:46.08elapsed 961%CPU (0avgtext+0avgdata 505216maxresident)k

	PR testsuite/66005
	gcc/
	* doc/install.texi: Document (optional) Perl usage for parallel
	testing of libgomp.
	libgomp/
	* testsuite/lib/libgomp.exp: 'flock' through stdout.
	* testsuite/flock: New.
	* configure.ac (FLOCK): Point to that if no 'flock' available, but
	'perl' is.
	* configure: Regenerate.

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