Add x86_64-linux-gnu emulator

I wanted a tiny scriptable meltdown proof way to run userspace programs
and visualize how program execution impacts memory. It helps to explain
how things like Actually Portable Executable works. It can show you how
the GCC generated code is going about manipulating matrices and more. I
didn't feel fully comfortable with Qemu and Bochs because I'm not smart
enough to understand them. I wanted something like gVisor but with much
stronger levels of assurances. I wanted a single binary that'll run, on
all major operating systems with an embedded GPL barrier ZIP filesystem
that is tiny enough to transpile to JavaScript and run in browsers too.

https://justine.storage.googleapis.com/emulator625.mp4

libc/rand/rngset.c

@@ -22,16 +22,18 @@
 #include "libc/str/str.h"
 
 /**
- * Fills memory with random bytes.
+ * Fills memory with random bytes, e.g.
  *
- * @param seed can be rand64
- * @param reseed is number of bytes between seed() calls
- * @return buf
+ *   char buf[1024];
+ *   rngset(buf, sizeof(buf), rand64, -1);
+ *
+ * @param seed can be rand64() and is always called at least once
+ * @param reseed is bytes between seed() calls and -1 disables it
+ * @return original buf
  */
-void *rngset(void *buf, size_t size, uint64_t (*seed)(void), size_t reseed) {
+void *rngset(void *buf, size_t size, uint64_t seed(void), size_t reseed) {
   unsigned char *p;
   uint64_t i, x, state;
-  i = 0;
   p = buf;
   state = seed();
   for (i = 0; size - i >= sizeof(x); i += sizeof(x)) {