powerpc: introduce arch_enter_from_user_mode

powerpc: introduce arch_enter_from_user_mode

Implement the arch_enter_from_user_mode() hook required by the generic
entry/exit framework. This helper prepares the CPU state when entering
the kernel from userspace, ensuring correct handling of KUAP/KUEP,
transactional memory, and debug register state.

This patch contains no functional changes, it is purely preparatory for
enabling the generic syscall and interrupt entry path on PowerPC.

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/20260427122742.210074-4-mkchauras@gmail.com