Current documentation is not too clear on the GETPC usage.
In particular, when used outside the top level helper function
it causes unexpected behavior.

Signed-off-by: Emanuele Giuseppe Esposito <e.emanuelegiuseppe@gmail.com>
Message-id: 20201015095147.1691-1-e.emanuelegiuseppe@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>