Make STM32F4 vectors relocate properly

This fixes a regression introduced by https://github.com/mbedmicro/mbed/commit/14409cd7fd6bc903a9cf87c3dccbc854e895033d.  The old vector table is not located as 0x0, so the table was not relocated when NVIC_SetVector was called.  I've used the technique for relocation from the LPC43XX code, as the STM32F4 can also store its vectors in one of a couple of locations.