cpu: unified cortex-m base interrupt vector

cpu/cc2538/vectors.c

cpu/cc26x0/vectors.c

cpu/cortexm_common/include/vectors_cortexm.h

cpu/cortexm_common/ldscripts/cortexm_base.ld

cpu/cortexm_common/vectors_cortexm.c

cpu/ezr32wg/vectors.c

cpu/k22f/vectors.c

cpu/k60/vectors.c

cpu/k64f/vectors.c

cpu/kinetis_common/include/vectors_kinetis.h

cpu/kinetis_common/ldscripts/kinetis.ld

cpu/kw2xd/vectors.c

cpu/lm4f120/vectors.c

cpu/lpc1768/vectors.c

cpu/nrf51/vectors.c

cpu/nrf52/vectors.c

cpu/sam3/vectors.c

cpu/samd21/vectors.c

cpu/saml21/vectors.c

cpu/stm32f0/vectors.c