CSKY: Support two operands form for bloop.

gas/
        * config/tc-csky.c (csky_insn_info): Add member last_isize.
        (md_assemble): Assign value to csky_insn.last_isize.
        * testsuite/gas/csky/enhance_dsp.d: Test bloop's two operands form.
        * testsuite/gas/csky/enhance_dsp.s: Likewise.

opcodes/
        * csky-opc.h (csky_v2_opcodes): Add two operands form for bloop.