Adds missing fully qualified arm arch version.

[mlondono74]

When building for our platform got #error "unknown CPU architecture: ARM"
The fully qualified arm version for our platform which specifies the FPU is ARM_ARCH_7EM

$ echo | arm-none-eabi-gcc -dM -E -mcpu=cortex-m7 -mthumb -mfpu=fpv5-d16 - | grep ARCH
#define __ARM_ARCH_PROFILE 77
#define __ARM_ARCH_ISA_THUMB 2
#define __ARM_ARCH 7
#define ARM_ARCH_7EM 1
#define ARM_ARCH_EXT_IDIV 1

This was missing in cpu.hpp, hence the PR.

[biojppm]

Thanks!

[mlondono74]

Thank you for the great library!