Remove BSP_INTERRUPT_VECTOR_MIN and unconditionally let interrupt vector
numbers start with zero.
The BSP_INTERRUPT_VECTOR_MIN == 0 invariant was tested by the previous commit
and building all BSPs.
Update #3269.
Set the VBAR to the vector table in the start section before
bsp_start_hook_0() is called to earlier handle exceptions in RTEMS.
Set the VBAR to the normal vector table in start.S for the main
processor. Secondary processors set it in bsp_start_hook_0().
Update #4202.
This breaks AArch32-specific portions of the ARM GPT driver into their
own file so that the generic code can be moved for reuse by other
architectures.
