powerpc/ftrace: Regroup PPC64 specific operations in ftrace_mprofile.S

CONFIG_MPROFILE_KERNEL is only for PPC64 and ftrace_mprofile.o
is build on PPC64 only when CONFIG_MPROFILE_KERNEL is defined.

Move saving of r0 inside #ifdef PPC64

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/619dfb672bf4f1b777a4b3f8b4f14e637fea2716.1645099283.git.christophe.leroy@csgroup.eu