s390/boot: avoid unnecessary zeroing of .bss section

.bss section is a part of the decompressor's image now, linker fills it
with zeros already. No need do it with memset additionally.

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