s390/decompressor: get rid of .bss usage

s390/decompressor: get rid of .bss usage

Using .bss in early code should be avoided. It might overlay initrd
image or not yet be initialized. Clean up the last couple of places in
the decompressor's code where .bss is used and enfore no .bss usage
check on boot/compressed/misc.c. In particular:
- initializing free_mem_ptr and free_mem_end_ptr with values guarantee
that these variables won't end up in the .bss section.
- define STATIC_RW_DATA to go into .data section.

Reviewed-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>