Ensure we call into the host C library open/close/read/write
independently of which embedded C library the code is built with.
We do this by:
a) Using the native simulator nsi_host* trampolines when we just want
to call straight into the host libC.
b) Building in the native_simulator runner context (and therefore with
the host C library) two functions which we call from the embedded
side.
Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>
6ef08d1221