cmd/go/internal: configure go tool workflow for loong64

Contributors to the loong64 port are:
  Weining Lu <luweining@loongson.cn>
  Lei Wang <wanglei@loongson.cn>
  Lingqin Gong <gonglingqin@loongson.cn>
  Xiaolin Zhao <zhaoxiaolin@loongson.cn>
  Meidan Li <limeidan@loongson.cn>
  Xiaojuan Zhai <zhaixiaojuan@loongson.cn>
  Qiyuan Pu <puqiyuan@loongson.cn>
  Guoqi Chen <chenguoqi@loongson.cn>

This port has been updated to Go 1.15.6:
  https://github.com/loongson/go

Updates #46229

Change-Id: I6b537a7d842b0683586917fe7ea7cd4d70d888de
Reviewed-on: https://go-review.googlesource.com/c/go/+/342308
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/imports/build.go

@@ -328,6 +328,7 @@ var KnownArch = map[string]bool{
 	"mips64le":    true,
 	"mips64p32":   true,
 	"mips64p32le": true,
+	"loong64":     true,
 	"ppc":         true,
 	"riscv":       true,
 	"riscv64":     true,

src/cmd/go/internal/work/exec.go

@@ -2654,6 +2654,8 @@ func (b *Builder) gccArchArgs() []string {
 		} else if cfg.GOMIPS == "softfloat" {
 			return append(args, "-msoft-float")
 		}
+	case "loong64":
+		return []string{"-mabi=lp64d"}
 	case "ppc64":
 		if cfg.Goos == "aix" {
 			return []string{"-maix64"}