powerpc/ftrace: Remove ftrace_32.S

Functions in ftrace_32.S are common with PPC64.

Reuse the ones defined for PPC64 with slight modification
when required.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Squash in fixup diff from Christophe]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/5e837fc190504c4ef834272e70d60ae33f175d49.1640017960.git.christophe.leroy@csgroup.eu