powerpc/ftrace: Prepare ftrace_64_mprofile.S for reuse by PPC32

PPC64 mprofile versions and PPC32 are very similar.

Modify PPC64 version so that if can be reused for PPC32.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/82a732915dc71ee766e31809350939331944006d.1640017960.git.christophe.leroy@csgroup.eu