Tier 3 support for base targets used by Alpine Linux

text/0000-alpine-targets.md

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+- Start Date: 2022-10-05
+- RFC PR: (leave this empty)
+- Nix Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Add the following targets as tier 3 supported platforms:
+  * `aarch64-unknown-linux-musl`
+  * `arm-unknown-linux-musleabihf`
+  * `armv7-unknown-linux-musleabihf`
+  * `powerpc-unknown-linux-musl`
+  * `powerpc64-unknown-linux-musl`
+  * `riscv64gc-unknown-linux-musl`
+  * `s390x-unknown-linux-musl`
+
+# Motivation
+[motivation]: #motivation
+
+The listed targets are used by Alpine Linux for supported architectures (Alpine
+creates its own targets, but they are based on the listed ones), except the ones
+already supported in nix (`x86_64-unknown-linux-musl`, `i686-unknown-linux-musl`),
+and `powerpc64le-unknown-linux-musl`, which is not supported by the libc crate.
+
+Nix failures to compile block at least 10 packages from getting built
+for s390x and/or riscv64 architectures, my quick GitLab search for "nix crate" shows.
+This should help to monitor potential changes in nix for breaking these architectures.
+
+# Detailed design
+[design]: #detailed-design
+
+N/A
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Possibly increased needed maintainership time. However, failing tier 3 CI
+should not be blocking code from inclusion.
+
+# Alternatives
+[alternatives]: #alternatives
+
+No new targets.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+N/A