cmd/internal/obj/loong64: optimize the code logic of jump instructions
If p.To.Sym is nil, that means we can get the target offset from
p.To.Target().pc - c.pc，only when p.To.Sym is not nil, we need relocation
to get the true address of target symbol.
Change-Id: Ied52f675c6aa6e8fb8d972b7699f5cadd1ecb268
Reviewed-on: https://go-review.googlesource.com/c/go/+/565627
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Knyszek <mknyszek@google.com> Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn> Reviewed-by: Qiqi Huang <huangqiqi@loongson.cn> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com> Reviewed-by: abner chenc <chenguoqi@loongson.cn> Reviewed-by: Meidan Li <limeidan@loongson.cn>