Replace open-coded toc-relative address calculation with helper macros,
commit dab3b8f4 ("powerpc/64: asm use consistent global variable
declaration and access") made similar conversions already but missed
this one.

This allows data addressing model to be changed more easily.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230203113858.1152093-2-npiggin@gmail.com