powerpc: Add support for relative exception tables

This halves the exception table size on 64-bit builds, and it allows
build-time sorting of exception tables to work on relocated kernels.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Minor asm fixups and bits to keep the selftests working]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>