The `mipsel` `target_arch` was introduced with the initial MIPSel
support (rust-lang/rust@82ec1aef29),
but was subsequently removed with implementation of the Flexible Target
Specification (Rust RFC 0131,
rust-lang/rust@3a8f4ec32a).
This is the only remaining instance in rustc. All others are in the libc
repo, and are fixed in rust-lang/libc@b3676593f6.