s390/preempt: Optimize __preempt_count_dec_and_test()

s390/preempt: Optimize __preempt_count_dec_and_test()

Provide an inline assembly using alternatives to avoid the need of a
base register due to relocatable lowcore when adding or subtracting
small constants from preempt_count.

Main user is preempt_enable(), which subtracts one from preempt_count
and tests if the result is zero.

With this the generated code changes from

  1000b8:       a7 19 00 00             lghi    %r1,0
  1000bc:       eb ff 13 a8 00 6e       alsi    936(%r1),-1
  1000c2:       a7 54 00 05             jnhe    1000cc <__rcu_read_unlock+0x14>

to something like this:

  1000b8:       eb ff 03 a8 00 6e       alsi    936,-1
  1000be:       a7 54 00 05             jnhe    1000c8 <__rcu_read_unlock+0x10>

Kernel image size is reduced by 45kb (bloat-o-meter -t, defconfig, gcc15).

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>