HACKING: List areas where we may rely on impdef C behaviour

Add a section to HACKING saying which version of the C spec
we use and describing the bits of implementation defined C
compiler behaviour which C code in QEMU is allowed to rely on.

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