Make malloc scalable on all platforms

It turns out sched_getcpu() didn't work on many platforms. So the system
call now has tests and is well documented. We now employ new workarounds
on platforms where it isn't supported in our malloc() implementation. It
was previously the case that malloc() was only scalable on Linux/Windows
for x86-64. Now the other platforms are scalable too.

examples/nproc.c

@@ -0,0 +1,15 @@
+#if 0
+/*─────────────────────────────────────────────────────────────────╗
+│ To the extent possible under law, Justine Tunney has waived      │
+│ all copyright and related or neighboring rights to this file,    │
+│ as it is written in the following disclaimers:                   │
+│   • http://unlicense.org/                                        │
+│   • http://creativecommons.org/publicdomain/zero/1.0/            │
+╚─────────────────────────────────────────────────────────────────*/
+#endif
+#include <cosmo.h>
+#include <stdio.h>
+
+int main(int argc, char *argv[]) {
+  printf("%d\n", __get_cpu_count());
+}