E407/demo.c (vParTestToggleLED): only blink LED if invoked by 1st task

This removes the erratic blinking.
E407/demo.c: clean up builtin "shadows"
E407/: move libc "shadow" functions from demo.c to libc.c

The mystery is now solved: they're used to back up builtins when someone takes
a pointer or when the builtin falls back to the unoptimized general case.