gdb/csky just return type from csky_vector_type() for vector resgisters

gdb/csky just return type from csky_vector_type() for vector resgisters

Some gdb stubs may not describe the type for vector registers in the
tdesc-xml and only send bitsize="128", gdb can't deal with a reg
with default type int with bitsize==128. So Just return csky_vector_type()
for vector resgisters.