bsp/leon3: Replace the define LEON3_MP_IRQ with a weakly linked variable

The LEON3_MP_IRQ define is used to pick the IRQ to be used by the
shared memory driver and for inter-processor interrupts. On some LEON3
systems, for example the GR712RC, the default value of 14 is not suitable.
To make this value configurable from the application, it is replaced with
a weakly linked variable that can be overridden from the application.
Fix assertion "BUF != NULL" for i386

 BSP_uart_termios_write_com1 and BSP_uart_termios_write_com2