Patch sets correct jmp_buf size for armv6-m conforming to implementation in setjmp.S

FYI a table of cortex architectures:
__ARM_ARCH_6M__ cortex-m0/m0+/m1 no fpu option
__ARM_ARCH_7M__ cortex-m3 no fpu option
__ARM_ARCH_7EM__ cortex-m4 optional fpu
check using __ARM_FP