x86/vdso: Prepare for robust futex unlock support

There will be a VDSO function to unlock non-contended robust futexes in
user space. The unlock sequence is racy vs. clearing the list_pending_op
pointer in the task's robust list head. To plug this race the kernel needs
to know the critical section window so it can clear the pointer when the
task is interrupted within that race window. The window is determined by
labels in the inline assembly.

Add these symbols to the vdso2c generator and use them in the VDSO VMA code
to update the critical section addresses in mm_struct::futex on (re)map().

The symbols are not exported to user space, but available in the debug
version of the vDSO.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Link: https://patch.msgid.link/20260602090535.828312645@kernel.org

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