lib.systems.examples: canonicalize MIPS triples

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.

lib/systems/examples.nix

@@ -91,25 +91,23 @@ rec {
     config = "mipsel-unknown-linux-gnu";
   } // platforms.fuloong2f_n32;
 
-  # MIPS ABI table transcribed from here: https://wiki.debian.org/Multiarch/Tuples
-
   # can execute on 32bit chip
-  mips-linux-gnu                = { config = "mips-linux-gnu";                } // platforms.gcc_mips32r2_o32;
-  mipsel-linux-gnu              = { config = "mipsel-linux-gnu";              } // platforms.gcc_mips32r2_o32;
-  mipsisa32r6-linux-gnu         = { config = "mipsisa32r6-linux-gnu";         } // platforms.gcc_mips32r6_o32;
-  mipsisa32r6el-linux-gnu       = { config = "mipsisa32r6el-linux-gnu";       } // platforms.gcc_mips32r6_o32;
+  mips-linux-gnu                = { config = "mips-unknown-linux-gnu";                } // platforms.gcc_mips32r2_o32;
+  mipsel-linux-gnu              = { config = "mipsel-unknown-linux-gnu";              } // platforms.gcc_mips32r2_o32;
+  mipsisa32r6-linux-gnu         = { config = "mipsisa32r6-unknown-linux-gnu";         } // platforms.gcc_mips32r6_o32;
+  mipsisa32r6el-linux-gnu       = { config = "mipsisa32r6el-unknown-linux-gnu";       } // platforms.gcc_mips32r6_o32;
 
   # require 64bit chip (for more registers, 64-bit floating point, 64-bit "long long") but use 32bit pointers
-  mips64-linux-gnuabin32        = { config = "mips64-linux-gnuabin32";        } // platforms.gcc_mips64r2_n32;
-  mips64el-linux-gnuabin32      = { config = "mips64el-linux-gnuabin32";      } // platforms.gcc_mips64r2_n32;
-  mipsisa64r6-linux-gnuabin32   = { config = "mipsisa64r6-linux-gnuabin32";   } // platforms.gcc_mips64r6_n32;
-  mipsisa64r6el-linux-gnuabin32 = { config = "mipsisa64r6el-linux-gnuabin32"; } // platforms.gcc_mips64r6_n32;
+  mips64-linux-gnuabin32        = { config = "mips64-unknown-linux-gnuabin32";        } // platforms.gcc_mips64r2_n32;
+  mips64el-linux-gnuabin32      = { config = "mips64el-unknown-linux-gnuabin32";      } // platforms.gcc_mips64r2_n32;
+  mipsisa64r6-linux-gnuabin32   = { config = "mipsisa64r6-unknown-linux-gnuabin32";   } // platforms.gcc_mips64r6_n32;
+  mipsisa64r6el-linux-gnuabin32 = { config = "mipsisa64r6el-unknown-linux-gnuabin32"; } // platforms.gcc_mips64r6_n32;
 
   # 64bit pointers
-  mips64-linux-gnuabi64         = { config = "mips64-linux-gnuabi64";         } // platforms.gcc_mips64r2_64;
-  mips64el-linux-gnuabi64       = { config = "mips64el-linux-gnuabi64";       } // platforms.gcc_mips64r2_64;
-  mipsisa64r6-linux-gnuabi64    = { config = "mipsisa64r6-linux-gnuabi64";    } // platforms.gcc_mips64r6_64;
-  mipsisa64r6el-linux-gnuabi64  = { config = "mipsisa64r6el-linux-gnuabi64";  } // platforms.gcc_mips64r6_64;
+  mips64-linux-gnuabi64         = { config = "mips64-unknown-linux-gnuabi64";         } // platforms.gcc_mips64r2_64;
+  mips64el-linux-gnuabi64       = { config = "mips64el-unknown-linux-gnuabi64";       } // platforms.gcc_mips64r2_64;
+  mipsisa64r6-linux-gnuabi64    = { config = "mipsisa64r6-unknown-linux-gnuabi64";    } // platforms.gcc_mips64r6_64;
+  mipsisa64r6el-linux-gnuabi64  = { config = "mipsisa64r6el-unknown-linux-gnuabi64";  } // platforms.gcc_mips64r6_64;
 
   muslpi = raspberryPi // {
     config = "armv6l-unknown-linux-musleabihf";