s390/boot: cleanup decompressor header files

s390/boot: cleanup decompressor header files

[ Upstream commit 9c3205b2b062420c26b33924b910880889acf832 ]

Move declarations to appropriate header files. Instead of cryptic
casting directly assign struct vmlinux_info type to _vmlinux_info
linker script variable - wich it actually is.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Stable-dep-of: 22476f47b6b7 ("s390/boot: fix mem_detect extended area allocation")
Signed-off-by: Sasha Levin <sashal@kernel.org>