s390/boot: Use -D__DISABLE_EXPORTS

s390/boot: Use -D__DISABLE_EXPORTS

Files in the arch/s390/boot directory reuse logic from the rest of the
kernel by including certain C and assembly files from the kernel and lib
directories. Some of these included files contain EXPORT_SYMBOL directives.
For instance, arch/s390/boot/cmdline.c includes lib/cmdline.c, which
exports the get_option() function.

This inclusion triggers genksyms processing for the files in
arch/s390/boot, which is unnecessary and slows down the build.
Additionally, when KBUILD_SYMTYPES=1 is set, the generated symtypes data
contain exported symbols that are duplicated with the main kernel. This
duplication can confuse external kABI tools that process the symtypes data.

Address this issue by compiling the files in arch/s390/boot with
-D__DISABLE_EXPORTS.

Signed-off-by: Petr Pavlu <petr.pavlu@suse.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Link: https://lore.kernel.org/r/20250620154649.116068-1-petr.pavlu@suse.com
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>