s390/purgatory: disable branch profiling

Avoid linker error for randomly generated config file that
has CONFIG_BRANCH_PROFILE_NONE enabled and make it similar
to riscv, x86 and also to commit 4bf3ec38

 ("s390: disable
branch profiling for vdso").

Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>