feat: Add BPF triple constraint mapping

[tamird]

Add support for tier 3 targets bpfeb-unknown-none and bpfel-unknown-none (see https://github.com/rust-lang/rust/blob/f5e2df7/src/doc/rustc/src/platform-support.md?plain=1#L311-L312).

This is modeled after https://github.com/bazelbuild/rules_rust/pull/3507 and should probably be updated if/when https://github.com/bazelbuild/platforms/pull/131 is merged.

(please use rebase merge when landing this as the proper commit message is in the commit, rather than the PR description)

/cc @avrabe