In Nixpkgs, we assume that the "config" field is a canonicalized GNU
triple. I noticed that non-canonical values were being used here,
because the pkgsCross.mips64el-linux-gnu triples did not contain the
vendor field, but the pkgsCross.mips64el-linux-gnu.pkgsStatic did.
Here, I've run all the MIPS triples in lib.systems.examples through
config.sub to canonicalize them. I think this will avoid nasty
surprises in future.
Tested by building Nix and the bootstrap files for
pkgsCross.mips64el-linux-gnu.
e8d7d52fae