s390/vdso: Disable kstack erase

s390/vdso: Disable kstack erase

For some reason gcc 8, 9, 10, and 11 generate a dynamic relocation in
vdso.so.dbg if CONFIG_KSTACK_ERASE is enabled:

>> arch/s390/kernel/vdso/vdso.so.dbg: dynamic relocations are not supported
   make[3]: *** [arch/s390/kernel/vdso/Makefile:54: arch/s390/kernel/vdso/vdso.so.dbg] Error 1

$ readelf -rW arch/s390/kernel/vdso/vdso.so.dbg

Relocation section '.rela.dyn' at offset 0x15c0 contains 1 entry:
    Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
00000000000015f0  000000010000000b R_390_JMP_SLOT         0000000000000000 __sanitizer_cov_stack_depth + 0

Add $(DISABLE_KSTACK_ERASE) to vdso compile flags to fix this.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/r/202601070505.xQcLr5KV-lkp@intel.com/
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>