* [cpu] 增加ARCH_CPU_BIG_ENDIAN
- 之前RT_USING_BIG_ENDIAN的命名并不合理，予以替换为ARCH_CPU_BIG_ENDIAN
- 删除一处多余的ARCH_CPU_64BIT
- 整理了libcpu/Kconfig的格式

* [libcpu][Kconfig] Cortex-M4&7 select ARCH_ARM_CORTEX_FPU

* revert last commit