s390/alternatives: avoid using jgnop mnemonic

jgnop mnemonic is only available since binutils 2.36,
kernel minimal required version is 2.23. Stick to brcl
to avoid build errors.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Fixes: 4afeb670

 ("s390/alternatives: use instructions instead of byte patterns")
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>