In most cases, unwinding will use the frame pointer anyway, so it
doesn't make much of a difference, but for cases where it won't,
capture the actual stack pointer.

(In most cases on arm64, calling the RtlCaptureContext won't cause
anything extra to be pushed on the stack at that point anyway).
Signed-off-by: Martin Storsjo <martin@martin.st>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>