cmd/internal/obj: add stmt prologueEnd to DWARF for loong64

cmd/internal/obj: add stmt prologueEnd to DWARF for loong64

Change-Id: Ib477b2402e32ebb3c7623caf4e9d62f3afdee92f
Reviewed-on: https://go-review.googlesource.com/c/go/+/556615
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Qiqi Huang <huangqiqi@loongson.cn>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>