Remove the include of setjmp.h from the cpu.h of target-alpha
and target-ppc. This is unnecessary because cpu-defs.h already
includes this header; this change brings these two targets
into line with all the rest.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>