s390/decompressor: support extra debug flags

Since decompressor cflags and aflags are made from scratch add extra
debug flags when CONFIG_DEBUG_INFO is enabled.

Also adds support for CONFIG_DEBUG_INFO_DWARF4 option.

Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>