Add build target for Linux loong64

[wxpppp]

What this PR does / why we need it:
The LoongArch architecture (LoongArch) is an Instruction Set Architecture (ISA) that has a RISC style, that is being used by more and more users.
Which issue(s) this PR fixes:
This patch supports building Grafana plugins on machines with the loong64/loongarch64 architecture.

Fixes #

Special notes for your reviewer:

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[xnyo]

Hello and thank you for contributing to Grafana!

In order to add support for plugins on loong64 architecture, Grafana itself also needs to be compiled for loong64. We currently only offer amd64 and arm64 binaries for Linux.

We encourage you to open an issue at https://github.com/grafana/grafana to request and discuss adding support for loong64 to Grafana.

In the meantime, if you are building Grafana for loong64 from source, you can still build plugins for such architecture by adding a custom target in your plugin's Magefile, like so:

//go:build mage
// +build mage

package main

import (
	"github.com/magefile/mage/mg"

	// mage:import
	build "github.com/grafana/grafana-plugin-sdk-go/build"
)

func BuildAll() error {
	mg.Deps(build.BuildAll)

	var b build.Build
	if err := build.SetBeforeBuildCallback(func(cfg build.Config) (build.Config, error) {
		cfg.OS = "linux"
		cfg.Arch = "loong64"
		return cfg, nil
	}); err != nil {
		return err
	}
	return b.Backend()
}

// Default configures the default target.
var Default = BuildAll

[wxpppp]

Thank you for your detailed reply. I will submit a new PR after Grafana supports loongarch.