!signal: move signal handling to kernel

[etiaro]

Description

• remove default handlers, signal_common and _signals_init from libphoenix
• switch from signalHandle syscall to signalAction
• add handler address argument to signal_trampoline
• remove phx-posix signal numbers translation
• remove signalPost syscall as duplicate to sys_tkill

test_results

Motivation and Context

JIRA: RTOS-736

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: libc: add more signal tests phoenix-rtos-tests#410
• Tested by hand on:

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).
• !syscalls: move signal handling to kernel
• I will merge this PR by myself when appropriate.

[gemini-code-assist]

Summary of Changes

Hello @etiaro, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request fundamentally refactors the signal handling mechanism by migrating core signal management logic from the libphoenix userspace library into the kernel. This change centralizes signal disposition, handler registration, and signal mask management within the operating system's core, simplifying the userspace signal API and removing the need for userspace signal number translation.

Highlights

• Kernel-centric Signal Handling: The primary goal of this PR is to migrate core signal management logic from the libphoenix userspace library into the kernel, centralizing signal disposition and handler registration.
• Streamlined Signal Delivery: The _signal_trampoline assembly code (arch/arm/signal.S) has been updated to directly invoke the userspace signal handler address provided by the kernel, removing an intermediate userspace handler call.
• Updated Signal API: The signalHandle syscall has been replaced with a new signalAction syscall (include/sys/threads.h), which now accepts sigaction structures and a trampoline address, aligning with more standard POSIX signal disposition mechanisms.
• Removal of Userspace Signal Management: libphoenix no longer maintains its own signal handler tables, default signal actions, or the complex Phoenix-to-POSIX signal number translation arrays. This logic is now expected to reside within the kernel.
• Simplified Userspace Signal Functions: Standard signal functions such as signal(), sigaction(), sigprocmask(), sigsuspend(), raise(), kill(), and killpg() in libphoenix (signal/signal.c) have been refactored to directly interact with the kernel's new signal API, passing raw signal numbers without userspace translation.

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[gemini-code-assist]

Code Review

This pull request refactors signal handling by moving signal handling to the kernel. A critical issue was identified in the new implementation of sigprocmask related to SIG_UNBLOCK and SIG_SETMASK logic, and a fix has been suggested.