powerpc: Switch to relative jump labels

Convert powerpc to relative jump labels.

Before the patch, pseries_defconfig vmlinux.o has:
9074 __jump_table  0003f2a0  0000000000000000  0000000000000000  01321fa8  2**0

With the patch, the same config gets:
9074 __jump_table  0002a0e0  0000000000000000  0000000000000000  01321fb4  2**0

Size is 258720 without the patch, 172256 with the patch.
That's a 33% size reduction.

Largely copied from commit c296146c ("arm64/kernel: jump_label:
Switch to relative references")

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/828348da7868eda953ce023994404dfc49603b64.1616514473.git.christophe.leroy@csgroup.eu