csky: bugfix for '2b - 1b' compile error in abiv2.

This bug is come from binutils/gas/config/tc-csky.c:
is_imm_over_orange() failed. Gas couldn't recognize the bt is 16/32
opcode, and it cause error when parse '2b - 1b'.

Change-Id: I01a13b910954c57cc856c7f17f32d72c3d6e4cfe
Signed-off-by: Guo Ren <ren_guo@c-sky.com>