native_simulator: Allow to pass extra options for localizing symbols

Some libraries (like Openthread's spinel code) define their
API as externally linkable. This will make those symbols
remain as externally linkable by default after the
Zephyr build has produced the native simulator library
(MCU code).
When building an AMP native_simulator executable with
several MCUs each including these, the linker will see
those symbols as still linkable and duplicated, and
throw an error.

So let's give the option for users/developers of those
libraries to define extra symbols they want
to localize before assembling the final executable.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>