s390/purgatory: Use -D__DISABLE_EXPORTS

s390/purgatory: Use -D__DISABLE_EXPORTS

The object files in purgatory do not export symbols, so disable exports
for all the object files, not only sha256.o, with -D__DISABLE_EXPORTS.

This fixes a build failure with CONFIG_GENDWARFKSYMS, where we would
otherwise attempt to calculate symbol versions for purgatory objects and
fail because they're not built with debugging information:

error: gendwarfksyms: process_module: dwarf_get_units failed: no debugging information?
make[5]: *** [../scripts/Makefile.build:207: arch/s390/purgatory/string.o] Error 1
make[5]: *** Deleting file 'arch/s390/purgatory/string.o'

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202502120752.U3fOKScQ-lkp@intel.com/
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lore.kernel.org/r/20250213211614.3537605-2-samitolvanen@google.com
Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>