Make terminal ui binaries work well everywhere

Here's some screenshots of an emulator tui program that was compiled on
Linux, then scp'd it to Windows, Mac, and FreeBSD.

https://justine.storage.googleapis.com/blinkenlights-cmdexe.png
https://justine.storage.googleapis.com/blinkenlights-imac.png
https://justine.storage.googleapis.com/blinkenlights-freebsd.png
https://justine.storage.googleapis.com/blinkenlights-lisp.png

How is this even possible that we have a nontrivial ui binary that just
works on Mac, Windows, Linux, and BSD? Surely a first ever achievement.

Fixed many bugs. Bootstrapped John McCarthy's metacircular evaluator on
bare metal in half the size of Altair BASIC (about 2.5kb) and ran it in
emulator for fun and profit.

libc/intrin/pcmpgtb.c

@@ -23,6 +23,8 @@
 /**
  * Compares signed 8-bit integers w/ greater than predicate.
  *
+ * Note that operands can be xor'd with 0x80 for unsigned compares.
+ *
  * @param 𝑎 [w/o] receives result
  * @param 𝑏 [r/o] supplies first input vector
  * @param 𝑐 [r/o] supplies second input vector