[CSKY] Add inline asm constraints and related codegen support

There are kinds of inline asm constraints and corresponding register class or register as following.

 'b': mGPRRegClass
 'v': sGPRRegClass
 'w': sFPR32RegClass or sFPR64RegClass
 'c': C register
 'z': R14 register
 'h': HI register
 'l': LO register
 'y': HI or LO register

It also adds codegen test for inline-asm including constraints, clobbers and abi names.