fix: scope Rust compiler toolchains to declared target triples

[dzbarsky]

Calling declare_rustc_toolchains in a workspace package currently breaks wildcard cquery when the target platform is outside target_triples. Add one configurable alias matching the declared triples to every generated toolchain().target_settings, preserving the existing BPF, bootstrap, and channel restrictions. With unsupported platforms excluded from toolchain resolution, target_triple can default to the first declared triple and rust_std can default to empty_stdlib. The empty standard library avoids analyzing source-built standard libraries for unsupported platforms. Reject empty target_triples and document registration precedence.

Validation: all eight Bazel 9 analysis tests pass, covering both declared musl triples, GNU fallback, bootstrap and BPF selection, and unsupported-platform analysis of every generated target, including a source-only declaration. Wildcard cquery //rs/toolchains/tests/... passes on an excluded GNU platform with its execution platform registered. Confirmed the original code reproduces the reported select error; buildifier formatting/lint and git diff --check pass.

Fixes #246.

-zbarskybot