Makefile: use buildx when GOOS/GOARCH are specified

[maxenglander]

Addresses #12108

Right now it's not very easy to build Docker images on Mac M1, unless I'm missing something obvious.

If I try to build lite images for example, it will kick off a docker build, which will try to build arm64/aarch64 Docker images. This usually fails when the Docker build tries to download dependencies for that architecture, and the providers of those dependencies don't support it.

Trying figure out how to resolve arm64 dependencies is probably a good idea for the future, but this PR just provides a work-around by making it easier to create amd64 Docker images from Mac M1. buildx is kind of slow, but it's better than nothing. And I'm able to run amd64 images on Mac M1 thanks to rosetta.

This PR changes the make docker_* target so that it uses docker buildx to create cross-platform builds when either GOOS or GOARCH are different from, respectively,GOHOSTOS or GOHOSTARCH.

So for example, on my M1 Mac, I can build images for linux/amd64 like this:

$ GOOS=linux GOARCH=amd64 make docker_lite_mysql80

[vitess-bot]

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[deepthi]

LGTM, nice little fix.