gdb/csky complete csky_dwarf_reg_to_regnum

gdb/csky complete csky_dwarf_reg_to_regnum

For csky arch, the correspondence between Dwarf registers and GDB
registers are as follows:
dwarf regnos 0~31 ==> gdb regs r0~r31
dwarf regno  CSKY_HI_REGNUM(36) ==> gdb reg hi
dwarf regno  CSKY_LO_REGNUM(37) ==> gdb reg hi
dwarf regno  CSKY_PC_REGNUM(72) ==> gdb reg pc
dwarf regnos FV_PSEUDO_REGNO_FIRST(74)~FV_PSEUDO_REGNO_LAST(201)
==>
gdb regs s0~s127 (pseudo regs for float and vector regs)

other dwarf regnos have no corresponding gdb regs to them.