s390: Replace __ASSEMBLY__ with __ASSEMBLER__ in uapi headers

s390: Replace __ASSEMBLY__ with __ASSEMBLER__ in uapi headers

__ASSEMBLY__ is only defined by the Makefile of the kernel, so
this is not really useful for uapi headers (unless the userspace
Makefile defines it, too). Let's switch to __ASSEMBLER__ which
gets set automatically by the compiler when compiling assembler
code.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Link: https://lore.kernel.org/r/20250611140046.137739-2-thuth@redhat.com
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>