s390/boot: cleanup decompressor header files

s390/boot: cleanup decompressor header files

[ Upstream commit 1243fce04b57ce19d0d8a032605e2bbf68784e1c ]

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: 2db7e7c4b86f ("s390/boot: fix mem_detect extended area allocation")
Signed-off-by: Sasha Levin <sashal@kernel.org>