Improve alloca() memory safety

Now that all the bugs have been wormed out of the ASAN memory module we
can successfully check for underruns on large stack allocations.

examples/hello.c

@@ -9,6 +9,8 @@
 #endif
 #include "libc/stdio/stdio.h"
 
+STATIC_YOINK("mmap");  // TODO: fix bandaid for MODE=asan
+
 int main() {
   printf("%s\n", "hello world");
   return 0;