arm: generate thumb stub if is thumb mode

[iDings]

arm have thumb state and arm state, when pc is last bit is 1 which indicate that current is thumb state, on thumb state,
stub instruction should use thumb instructions.

https://www.embedded.com/introduction-to-arm-thumb/

[iDings]

#28 应该是类似的问题，这个跳转的话跟__thumb__的一样，但是有个疑问是为什么__thumb__的时候要跳过前面两个16bit的thumb指令?

cpp-stub/src/stub.h

Lines 62 to 69 in 41c1a8e

	char *f = (char *)clearBit0;\
	if (clearBit0 % 4 != 0) {\
	*(uint16_t *)&f[0] = 0xbe00;\
	}\
	*(uint16_t *)&f[2] = 0xf8df;\
	*(uint16_t *)&f[4] = 0xf000;\
	*(uint16_t *)&f[6] = (uint16_t)(fn_stub & 0xffff);\
	*(uint16_t *)&f[8] = (uint16_t)(fn_stub >> 16);\

[iDings]

这个改动在rtthread上验证是ok的