osthread: divides OS-related code over *_impl modules

[denizzzka]

What is happening is a part of the process of separating code from different platforms into different modules. This is latest PR on this.

I decided to leave callWithStackShell() as it is (without pulling it apart into *_impl modules) because it’s already good displays OS + ABI features and pulling it apart will worsen its readability. If it gets in the way in the future we'll do something about it.

All other version() branches related to different OSes were abolished.

Previous PRs:

#23150
#23361
#23439
#23447
#23460
#23476
#23481
#23487
#23488
#23516
#23749

[github-actions]

DMD perf check

No differences above the noise thresholds.

All measurements

Metric	Base	PR	Δ
compile hello.d (instr)	214.9 M	214.9 M	+0.001%
compile hello.d -O -release (instr)	233.2 M	233.2 M	+0.001%
compile Phobos (instr)	5,123.8 M	5,122.7 M	-0.021%
compile Phobos codegen (instr)	1,472.7 M	1,472.4 M	-0.020%
compile vibe.d (instr)	15,131.1 M	15,131.6 M	+0.003%
dmd binary size (stripped)	6.86 MB	6.86 MB	0.00%
hello binary size (stripped)	0.72 MB	0.72 MB	0.00%
peak RSS (compile hello.d)	43.40 MB	43.55 MB	+0.36%
peak RSS (compile Phobos)	619.1 MB	617.4 MB	-0.27%
peak RSS (compile vibe.d)	1918 MB	1917 MB	-0.06%
compile dmd itself (wall)	9.5 s	9.5 s	+0.12%
compile hello.d (wall)	51.6 ms	50.2 ms	-2.63%
compile Phobos (wall)	1,205 ms	1,197 ms	-0.66%

_{10bf5e4 vs merge-base b41cb91 · about these metrics}

[denizzzka]

Ready to review