Provide static toolchains for each supported architecture (fallback)

Whilst it is understandable that you cannot provide a gazillion number of binaries for any combination of host triplets possible, it is however quite an effort for users to bootstrap Rust for a target without a pre-compiled binary (e.g. if there are none for this architecture, your libc is not supported, you are using libcxx, etc.). With Rust becoming ever more integral to the Linux ecosystem, having a working Rust tool chain is not a nice to have, but a necessity.

It would therefore be great to have something to work with, regardless of which architecture, libc and libcxx is being used. This can be achieved by providing:

• a static tool chain
• for the basic features set (no CPU extensions)
• for each supported architecture

Since you cannot bootstrap Rust without Rust, this would make the process significantly easier for users, because it would remove the necessity to bootstrap Rust yourself altogether.

No files, tests, or entry points are identified. Start by mapping Rust's supported architectures and the existing toolchain distribution and bootstrap process; define the required static artifacts, supported feature set, and validation needed for each architecture before implementation.