powerpc: Prepare to build with generic entry/exit framework

powerpc: Prepare to build with generic entry/exit framework

This patch introduces preparatory changes needed to support building
PowerPC with the generic entry/exit (irqentry) framework.

The following infrastructure updates are added:
 - Add a syscall_work field to struct thread_info to hold SYSCALL_WORK_* flags.
 - Provide a stub implementation of arch_syscall_is_vdso_sigreturn(),
   returning false for now.
 - Introduce on_thread_stack() helper to detect if the current stack pointer
   lies within the task’s kernel stack.

These additions enable later integration with the generic entry/exit
infrastructure while keeping existing PowerPC behavior unchanged.

No functional change is intended in this patch.

Signed-off-by: Mukesh Kumar Chaurasiya <mchauras@linux.ibm.com>
Tested-by: Samir M <samir@linux.ibm.com>
Tested-by: David Gow <davidgow@google.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260427122742.210074-3-mkchauras@gmail.com