boost-context changed its name for mips from "mips1" to "mips" in this
commit:

  boostorg/context@6edc818

however the native-build code to detect the local architecture still
reports "mips1":

  https://github.com/boostorg/boost/blob/67c074b249538cf441724f9bbb3929d0f6b4f333/boostcpp.jam#L637

Therefore native builds of boost-context on mips must specify
architecture= explicitly; without this you will get link failures
"undefined reference to `jump_fcontext`" in code that uses
boost-context.

Currently the "cross compile" codepath, which provides explicit
architecture/abi/address-model/binary-format/os parameters, is
prefixed by this comment:

```
  # TODO: make this unconditional
```

This commit does so, at least if `isMips`.

This commit is needed in order for native builds of nix to succeed on
mips.

The explanation for the conditional introduced by
61d9f20 is longer than a reasonable
inline comment should be.  It directed the reader to use `git blame`,
but that tends to bitrot.  Let's point the user to a specific nixpkgs
git hash.

This commit cannot be squashed into the previous commit, because a
commit cannot mention its own commit-hash (without performaing an
expensive double-sha1 preimage attack, of course).