signal fixups

[hexagonal-sun]

• arch: arm64: vdso: new
  Add a very basic VDSO module which allow the kernel to provide various
  functions to userspace, such as sig_return.

• arch: arm64: signal: use VDSO for sigreturn
  Support non SA_RESTORER sigactions by using the trampoline in the kernel
  VDSO.

• process: refactor signal handling and fix action table sharing
  This commit refactors the signal handling subsystem to address a bug in
  process creation and to improve the separation of concerns between
  signal delivery and signal disposition.

  1. Fix Action Table Sharing:
     Previously, the signal action table was wrapped in an Arc, causing
     forked child processes to inadvertently share signal dispositions with
     the parent. SignalActionState now contains a direct SigActionSet and
     implements Clone, ensuring that processes receive a private copy of the
     action table upon fork/clone.

  2. Decouple Signal Selection from Execution:
     The logic for selecting a pending signal has been moved from the
     disposition state into a new take_signal method on OwnedTask. This
     decouples the "taking" of a signal (respecting masks and priorities)
     from the "actioning" of that signal. This is a prerequisite for
     implementing ptrace.

  3. Various cleanup bits:

  • Renamed SignalState to SignalActionState to more accurately
    reflect that it manages signal dispositions.
  • Removed the pending signal set out of the action state and directly
    into the ThreadGroup and OwnedTask.
  • Renamed set_pending to set_signal for consistency.
  • Simplified the signal delivery loop in dispatch_userspace_task
    using the new take_signal API.