Support for armv7-unknown-linux-musl

[MPThLee]

Problem you are trying to solve

It's bit complicated, but I need armv7 build of rust docker image. But which is not supported for alpine linux image. (Because I'm trying to use docker on 32bit RPi)

So, I tried to find what sources of image and found that Docker image is built from this script which uses rustup.

And also, rustup only supports two musl targets. Because both are Highest host tools support for musl.

Solution you'd like

It's not easy as it is just Tier 2 support. But I'm hoping to see the arm/v7 build on rustup.

Notes

• Hope to support i686-unknown-linux-musl #2984