Support `arm-linux-musleabihf` and `aarch64-linux-musl` targets

[j8r]

arm-linux-musleabihf and aarch64-linux-musl targets will be needed to compile the crystal compiler and package it for Alpine Linux on ARM architectures.

Looking at Rust's targets might be useful.

We should also change the triple amd64-unknown-openbsd to x86_64-unknown-openbsd

[straight-shoota]

I don't think it makes much sense to keep this issue open. It originally started with different goals and the main one has been implemented for a long time.
We've had support for aarch64-linux-musl since 0.25 (#5861).

arm-linux-musleabihf is a different story. The focus has clearly shifted towards more modern architectures, so this is probably not that relevant.
We're open to contributions for any new target platform. But arm-linux-musleabihf is not a priority.