[stm32f0xx] Fix '072 vector table.
[stm32f0xx] Fix '072 vector table.

 The STM32F072 and STM32F070 have different vector tables.  We can't
 #ifdef on the existance of the constans like PVD_IRQn because they
 are enum values.  Right now, since we only have two supported variants,
 we're switching on the variant.  This will likely not scale.

travisg/lk,littlekernel/lk,littlekernel/lk,hollanderic/lkstuff,hollanderic/lkstuff,hollanderic/lkstuff,littlekernel/lk,travisg/lk,travisg/lk,travisg/lk,hollanderic/lkstuff,littlekernel/lk