cmd/internal/obj/loong64: switch Lookup function call to ABIInternal mode

cmd/internal/obj/loong64: switch Lookup function call to ABIInternal mode

CL 521790 has experimentally enabled RegABI support on Loong64, so it
is possible to switch the Lookup function call to ABIInternal mode.

Change-Id: I3ae053e20c0791efebe6b6bdc9a1550a11372bc2
Reviewed-on: https://go-review.googlesource.com/c/go/+/544435
Reviewed-by: Qiqi Huang <huangqiqi@loongson.cn>
Reviewed-by: WANG Xuerui <git@xen0n.name>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
Reviewed-by: Cherry Mui <cherryyz@google.com>