core: cpu: provide function to acquire ISR stack usage

core/include/arch/thread_arch.h

cpu/arm7_common/arm_cpu.c

cpu/atmega_common/thread_arch.c

cpu/cortexm_common/include/cpu.h

cpu/cortexm_common/thread_arch.c

cpu/cortexm_common/vectors_cortexm.c

cpu/msp430-common/cpu.c

cpu/native/native_cpu.c

cpu/x86/x86_thread_arch.c