add builds for rust compatible musl toolchains

[jungleraptor]

Adds a custom build of a arm-linux-musleabihf toolchain that is compatible with the corresponding rust toolchain.

The rust musl libc is based on version 1.1.24, which is incompatible with pre-built versions of this toolchain like the ones available from musl.cc

This build is based off of the following two projects:

• https://github.com/richfelker/musl-cross-make - canonical scripts for building musl cross toolchains
• https://github.com/rust-cross/rust-musl-cross - docker recipes for building a rust compatible musl toolchain

Design notes

• adds a makefile target to build the toolchain locally: make <target> and the resulting artifact will be in the root directory
• adds a GH actions CI pipeline to build and release the toolchain

[silverjam]

Would prefer if we use GitHub actions since this is a public repo-- we have access to s3 via the GitHub Actions AWS user/role that's setup in Terraform

[jungleraptor]

Would prefer if we use GitHub actions since this is a public repo-- we have access to s3 via the GitHub Actions AWS user/role that's setup in Terraform

The main reason I went for jenkins is because we need to build against a compatible glib as we're using in the target CI environments (2.28).

Not impossible in GHA but it's a lot less ergonomic then how we have it set up in jenkins.