Add steps for the NVIC_{S,G}etVector functions in the porting guide

The functions `NVIC_SetVector` and `NVIC_GetVector` need to be renamed with leading underscores `__NVIC_SetVector`, `__NVIC_SetVector` on a ported target (mbed OS CMSIS files), since the introduction of the CMSIS 5 NVIC virtualization.

The porting guide must be updated to included these steps.

Locate the porting guide and review its existing steps for CMSIS NVIC functions and the CMSIS 5 NVIC virtualization note. Update the guide to cover renaming NVIC_SetVector and NVIC_GetVector with leading underscores on ported mbed OS CMSIS targets; done means the required steps are documented clearly.

Written by the indexing model from the issue text.