boost: if isMips use the "cross compile" codepath unconditionally

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.

pkgs/development/libraries/boost/generic.nix

@@ -83,7 +83,9 @@ let
     "-sEXPAT_LIBPATH=${expat.out}/lib"
 
     # TODO: make this unconditional
-  ] ++ optionals (stdenv.hostPlatform != stdenv.buildPlatform) [
+  ] ++ optionals (stdenv.hostPlatform != stdenv.buildPlatform ||
+                  # required; see this line's `git blame` for an explanation
+                  (stdenv.hostPlatform.isMips && versionAtLeast version "1.79")) [
     "address-model=${toString stdenv.hostPlatform.parsed.cpu.bits}"
     "architecture=${if stdenv.hostPlatform.isMips64
                     then if versionOlder version "1.78" then "mips1" else "mips"