linker: make debug monitor isr symbol weak

It seems that currently it's impossible to create a custom
implementation for debug monitor exception without updating the vector
table (z_arm_debug_monitor maps to fault).
My proposition is to make this symbol weak, so that it can be overriden.

Signed-off-by: Piotr Jasiński <piotr.jasinski@nordicsemi.no>
1 file changed