x/build/cmd/releasebot: distribute official binaries for linux/mips64le

[XiaodongLoong]

I want to install golang on linux/mips64le platform, there is no tar ball avaliable:
url: https://storage.googleapis.com/golang/go1.15.3.linux-mips64le.tar.gz
http response:

<Error>
<Code>NoSuchKey</Code>
<Message>The specified key does not exist.</Message>
<Details>
No such object: golang/go1.15.3.linux-mips64le.tar.gz
</Details>
</Error>

But it support download on linux/amd64 linux/armv6l linux/arm64 linux/386 linux/ppc64le linux/s390x.
https://github.com/docker-library/golang/blob/0efc93637bf9773fcf3d3557a3012f6124a50d87/1.15/buster/Dockerfile

What version of Go are you using (go version)?

$ go version
1.15.3

Does this issue reproduce with the latest release?

What operating system and processor architecture are you using (go env)?

go env Output

$ go env

What did you do?

What did you expect to see?

There is a binary tar ball, and I can install golang on linux/mips64le platform.

What did you see instead?

[cagedmantis]

/cc @golang/release

[mengzhuo]

FYI
#35593