* introduce zigrt file. it contains only weak symbols so that
   multiple instances can be merged. it contains __zig_panic
   so that multiple .o files can call the same panic function.
 * remove `@setFnVisible` builtin and add @setGlobalLinkage builtin
   which is more powerful
 * add `@panic` builtin function.
 * fix collision of symbols with extern prototypes and internal
   function names
 * add stack protector safety when linking against libc. To add
   the safety mechanism without libc requires implementing
   Thread Local Storage. See #276