powerpc32, ftrace: save and restore mcount regs with macro

Impact: clean up

Use a macro to save and restore the registers for PowerPC32,
since that code is duplicated.

This is similar to the work done by Cyrill Gorcunov for the
mcount code in x86_64.

Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>