lib/systems/parse.nix: show respect where deserved

The eminent Donald E. Knuth should be recognized as having equal
standing with such entities as IBM, Apple, and the Personal
Computer.  We should acknowledge this by including him as a "vendor".

Also, `gnu-config` recognizes `mmix-knuth-*` triples (and in fact
requires `vendor="knuth"` when `cpu="mmix"`) -- so we sort of have
to.  But we should do it anyways.

lib/systems/parse.nix

@@ -221,6 +221,8 @@ rec {
   vendors = setTypes types.openVendor {
     apple = {};
     pc = {};
+    knuth = {};
+
     # Actually matters, unlocking some MinGW-w64-specific options in GCC. See
     # bottom of https://sourceforge.net/p/mingw-w64/wiki2/Unicode%20apps/
     w64 = {};