powerpc/64s: Move dcbt/dcbtst sequence into a macro

There's an almost identical code sequence to specify load/store access
hints in __copy_tofrom_user_power7(), copypage_power7() and
memcpy_power7().

Move the sequence into a common macro, which is passed the registers to
use as they differ slightly.

There also needs to be a copy in the selftests, it could be shared in
future if the headers are cleaned up / refactored.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240229122521.762431-1-mpe@ellerman.id.au