Support malloc() on bare metal

Your Actually Portable Executables now contains a simple virtual memory
that works similarly to the Linux Kernel in the sense that it maps your
physical memory to negative addresses. This is needed to support mmap()
and malloc(). This functionality has zero code size impact. For example
the MODE=tiny LIFE.COM executable is still only 12KB in size.

The APE bootloader code has also been simplified to improve readibility
and further elevate the elegance by which we're able to support so many
platforms thereby enhancing verifiability so that we may engender trust
in this bootloading process.

libc/runtime/exit2.c

@@ -21,11 +21,14 @@
 /**
  * Terminates process, ignoring destructors and atexit() handlers.
  *
- * @param rc is exit code ∈ [0,256)
+ * When running on bare metal, this function will reboot your computer
+ * by hosing the interrupt descriptors and triple faulting the system.
+ *
+ * @param exitcode is masked with 255
  * @asyncsignalsafe
  * @vforksafe
  * @noreturn
  */
-wontreturn void _exit(int rc) {
-  _Exit(rc);
+wontreturn void _exit(int exitcode) {
+  _Exit(exitcode);
 }