pthread: make pthread sched-related functions POSIX-compliant

[adamgreloch]

Description

Motivation and Context

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (refactoring, style fixes, git/CI config, submodule management, no code logic changes)

How Has This Been Tested?

• Already covered by automatic testing.
• New test added: damianloew/posix_tests
• Tested by hand on: riscv64

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing linter checks and tests passed.
• My changes generate no new compilation warnings for any of the targets.

Special treatment

• This PR needs additional PRs to work (list the PRs, preferably in merge-order).
  !proc: add schedGet and schedSet syscalls phoenix-rtos-kernel#806
• I will merge this PR by myself when appropriate.

[gemini-code-assist]

Code Review

This pull request introduces several POSIX-compliant scheduling functions, including sched_setparam, sched_getparam, sched_setscheduler, sched_getscheduler, and sched_rr_get_interval, and updates the pthread library to support scheduling parameter management. The review identifies several areas where error handling and return values need to be adjusted to strictly adhere to POSIX standards, specifically regarding error propagation and return values for scheduling functions.

[github-actions]

Unit Test Results

10 890 tests  ±0   10 220 ✅ ±0   52m 37s ⏱️ - 4m 4s
   680 suites ±0      670 💤 ±0 
     1 files   ±0        0 ❌ ±0 

Results for commit 2ca5da8. ± Comparison against base commit d0a2884.

♻️ This comment has been updated with latest results.