s390/vdso: correct CFI annotations of vDSO functions

Correct stack frame overhead for 31-bit vdso, which should be 96 rather
then 160. This is done by reusing STACK_FRAME_OVERHEAD definition which
contains correct value based on build flags. This fixes stack unwinding
within vdso code for 31-bit processes. While at it replace all hard coded
stack frame overhead values with the same definition in vdso64 as well.

Reviewed-by: Hendrik Brueckner <brueckner@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>