s390: use common bust_spinlocks()

s390 is the only architecture that is using own bust_spinlocks()
variant, while other arch-s seem to be OK with the common
implementation.

Heiko Carstens [1] said he would prefer s390 to use the common
bust_spinlocks() as well:
  I did some code archaeology and this function is unchanged since ~17
  years. When it was introduced it was close to identical to the x86
  variant. All other architectures use the common code variant in the
  meantime. So if we change this I'd prefer that we switch s390 to the
  common code variant as well. Right now I can't see a reason for not
  doing that

This patch removes s390 bust_spinlocks() and drops the weak attribute
from the common bust_spinlocks() version.

[1] lkml.kernel.org/r/20181025062800.GB4037@osiris
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>