powerpc: Fix register clobbering when accumulating stolen time

When running under a hypervisor that supports stolen time accounting,
we may call C code from the macro EXCEPTION_PROLOG_COMMON in the
exception entry path, which clobbers CR0.

However, the FPU and vector traps rely on CR0 indicating whether we
are coming from userspace or kernel to decide what to do.

So we need to restore that value after the C call

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>