s390/decompressor: allow preprocessor in piggy.o linker script

Rename vmlinux.scr to vmlinux.scr.lds.S and add it to build rules to
enable preprocessor for it.

Also add vmlinux.scr.lds to local .gitignore

Reviewed-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>