Add bpfel/bpfeb CPU constraint values

[tamird]

These names are used in clang (see https://github.com/llvm/llvm-project/blob/6bee6b2/llvm/include/llvm/TargetParser/Triple.h#L1154-L1157) and Rust (see https://github.com/rust-lang/rust/blob/f5e2df7/src/doc/rustc/src/platform-support.md?plain=1#L311-L312).

This addition is a prerequisite for teaching rules_rust about these constraints.

This is an alternative to https://github.com/bazelbuild/platforms/pull/99 which spelled these names incorrectly.

Closes https://github.com/bazelbuild/platforms/pull/99.

[tamird]

@gregestren could you please have a look at this and #123? The motivation between these two is very similar, and I'd like to follow @avrabe's lead on the proper way to plumb things in rules_rust.