s390/purgatory: Add -Wno-default-const-init-unsafe to KBUILD_CFLAGS

s390/purgatory: Add -Wno-default-const-init-unsafe to KBUILD_CFLAGS

Add -Wno-default-const-init-unsafe to purgatory KBUILD_CFLAGS, similar
to scripts/Makefile.extrawarn, since clang generates warnings for the
dummy variable in typecheck():

    CC      arch/s390/purgatory/purgatory.o
      arch/s390/include/asm/ptrace.h:221:9: warning: default initialization of an object of type 'typeof (regs->psw)' (aka 'const psw_t') leaves the object uninitialized [-Wdefault-const-init-var-unsafe]
        221 |         return psw_bits(regs->psw).pstate;
            |                ^
      arch/s390/include/asm/ptrace.h:98:2: note: expanded from macro 'psw_bits'
         98 |         typecheck(psw_t, __psw);                \
            |         ^
      include/linux/typecheck.h:11:12: note: expanded from macro 'typecheck'
         11 |         typeof(x) __dummy2; \
            |                   ^

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>