Macroize WRITE[-TO-STRING] defknowns, the latter missing a keyword

... which was recently corrected for one and not the other.
Remove a FIXME. lp#1026439 dealt with this.
Make cross-compilation a smidgen less noisy.
Rename *ALIEN-STACK* to *ALIEN-STACK-POINTER*

This eliminates one piddling annoyance from genesis.
Unbreak my previous change. Not sure how it worked.
Assign constant TLS indices to some important symbols.

And simplify related macrology.
Trivial change sb!vm::sym -> sb!vm:sym where it's an external symbol.