powerpc/code-patching: Move patch_exception() outside code-patching.c

patch_exception() is dedicated to book3e/64 is nothing more than
a normal use of patch_branch(), so move it into a place dedicated
to book3e/64.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/0968622b98b1fb51838c35b844c42ad6609de62e.1638446239.git.christophe.leroy@csgroup.eu