s390: Add -Wno-pointer-sign to KBUILD_CFLAGS_DECOMPRESSOR

If the decompressor is compiled with clang this can lead to the following
warning:

In file included from arch/s390/boot/startup.c:4:
...
In file included from ./include/linux/pgtable.h:6:
./arch/s390/include/asm/pgtable.h:2065:48: warning: passing 'unsigned long *' to parameter of type
      'long *' converts between pointers to integer types with different sign [-Wpointer-sign]
 2065 |                 value = __atomic64_or_barrier(PGSTE_PCL_BIT, ptr);

Add -Wno-pointer-sign to the decompressor compile flags, like it is also
done for the kernel. This is similar to what was done for x86 to address
the same problem [1].

[1] commit dca5203e ("x86/boot: Add -Wno-pointer-sign to KBUILD_CFLAGS")

Cc: stable@vger.kernel.org
Reported-by: Gerd Bayer <gbayer@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>