s390/alternatives: make use of asm_inline

This is the s390 version of commit 40576e5e ("x86: alternative.h:
use asm_inline for all alternative variants").

See commit eb111869 ("compiler-types.h: add asm_inline
definition") for more details.

With this change the compiler will not generate many out-of-line
versions for the three instruction sized arch_spin_unlock() function
anymore. Due to this gcc seems to change a lot of other inline
decisions which results in a net 6k text size growth according to
bloat-o-meter (gcc 9.2 with defconfig).
But that's still better than having many out-of-line versions of
arch_spin_unlock().

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>