arm64: add 'runtime constant' support

This implements the runtime constant infrastructure for arm64, 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.

[ Fixed up to deal with the big-endian case as per Mark Rutland ]

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>