These targets cover OpenWRT 15.05 devices, which use the soft float ABI
and the uclibc library. None of the other built-in mips targets covered
those devices (mips-gnu is hard float and glibc-based, mips-musl is
musl-based).
With this commit one can now build std for these devices using these
commands:
```
$ configure --enable-rustbuild --target=mips-unknown-linux-uclibc
$ make
```
cc #35673
