s390/compiler.h Fix sparse vs. hotpatch

sparse does not understand the s390 specific hotpatch attribute and
floods the log with messages like
include/uapi/linux/swab.h:92:8: error: attribute 'hotpatch': unknown attribute

Let's just dont use it, if __CHECKER__ is defined.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>