Fix Issue 19513 - Use sched_getaffinity(2) to get the number of CPU cores if abailable

[kubo39]

Currently std.parallelism.totalCPUs is implemented by
sysconf(_SC_NPROCESSORS_ONLN) on Posix platoform. However, on GNU/Linux,
usable number of processors may be restricted if a process runs in
container. In case it's better to use sched_getaffinity(2).

ref: http://man7.org/linux/man-pages/man1/nproc.1.html

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	19513	enhancement	Use sched_getaffinity(2) to get the number of CPU cores if available

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + phobos#7030"