[libc] clean up use of __builtin_popcountl in sched_getcpucount

[michaelrj-google]

In libc/src/sched/linux/sched_getcpucount.cpp we currently use __builtin_popcountl. Ideally we'd instead use cpp::popcount defined in libc/src/__support/CPP/bit.h.

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

1. Check that no other contributor has already been assigned to this issue. If you believe that no one is actually working on it despite an assignment, ping the person. After one week without a response, the assignee may be changed.
2. In the comments of this issue, request for it to be assigned to you, or just create a pull request after following the steps below. Mention this issue in the description of the pull request.
3. Fix the issue locally.
4. Run the test suite locally. Remember that the subdirectories under test/ create fine-grained testing targets, so you can e.g. use make check-clang-ast to only run Clang's AST tests.
5. Create a Git commit.
6. Run git clang-format HEAD~1 to format your changes.
7. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation. Mention this issue in the description of the pull request.

If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Michael Jones (michaelrj-google)

In [`libc/src/sched/linux/sched_getcpucount.cpp`](https://github.com/llvm/llvm-project/blob/main/libc/src/sched/linux/sched_getcpucount.cpp) we currently use `__builtin_popcountl`. Ideally we'd instead use `cpp::popcount` defined in [`libc/src/__support/CPP/bit.h`](https://github.com/llvm/llvm-project/blob/main/libc/src/__support/CPP/bit.h#L268).

[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Michael Jones (michaelrj-google)

In [`libc/src/sched/linux/sched_getcpucount.cpp`](https://github.com/llvm/llvm-project/blob/main/libc/src/sched/linux/sched_getcpucount.cpp) we currently use `__builtin_popcountl`. Ideally we'd instead use `cpp::popcount` defined in [`libc/src/__support/CPP/bit.h`](https://github.com/llvm/llvm-project/blob/main/libc/src/__support/CPP/bit.h#L268).

[liamsemeria]

i can work on this but i havent worked on an issue before it should be pretty straightforward right?

[michaelrj-google]

Yes, it should be fairly straightforward. Would you like me to assign the issue to you?

[liamsemeria]

ok cool. yeah that would be great!

[el-ev]

@liamsemeria Are you still working on this?

[liamsemeria]

@el-ev i was struggling to build libc on arm and i've had a couple things come up. feel free to work on it too

[michaelrj-google]

@el-ev I'll reassign to you for now.