s390: Add runtime constant support

Implement the runtime constant infrastructure for s390, allowing the
dcache d_hash() function to be generated using as a constant for hash
table address followed by shift by a constant of the hash index.

This is the s390 variant of commit 94a2bc0f ("arm64: add 'runtime
constant' support") and commit e3c92e81 ("runtime constants: add
x86 architecture support").

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>