powerpc: Introduce function emulate_math()

There are two invocations of do_mathemu() in traps.c. And the codes
in these two places are almost the same. Introduce a locale function
to eliminate the duplication. With this change we can also make sure
that in program_check_exception() the PPC_WARN_EMULATED is invoked for
the correctly emulated math instructions.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>