Add platform triple arm-unknown-linux-musleabi (#3821)

Add `arm-unknown-linux-musleabi` to the list of supported platform
triples in `SUPPORTED_T2_PLATFORM_TRIPLES`.

The `arm-unknown-linux-musleabi` triple is a valid Rust target supported
by the official Rust toolchain and is required for building Rust
projects targeting ARM devices with musl. This platform triple is also
recognized by popular crates in the ecosystem, such as the
https://crates.io/crates/nix/0.26.2, which explicitly supports this
target as T2.

Author: martin4861

rust/platform/triple_mappings.bzl

@@ -49,6 +49,7 @@ SUPPORTED_T2_PLATFORM_TRIPLES = {
     "aarch64-unknown-fuchsia": _support(std = True, host_tools = False),
     "aarch64-unknown-uefi": _support(std = True, host_tools = False),
     "arm-unknown-linux-gnueabi": _support(std = True, host_tools = True),
+    "arm-unknown-linux-musleabi": _support(std = True, host_tools = True),
     "armv7-linux-androideabi": _support(std = True, host_tools = False),
     "armv7-unknown-linux-gnueabi": _support(std = True, host_tools = True),
     "i686-linux-android": _support(std = True, host_tools = False),