FORTIFY_SOURCE: optimize

Don't do the fortify_source checks if we can determine, at
compile time, that the provided operation is safe.

This avoids silliness like calling fortify source on things like:

  size_t len = strlen("asdf");
  printf("%d\n", len);

and allows the compiler to optimize this code to:

  printf("%d\n", 4);

Defer to gcc's builtin functions instead of pointing our code
to the libc implementation.

Change-Id: I5e1dcb61946461c4afaaaa983e39f07c7a0df0ae

Conflicts:
	libc/include/string.h
	tests/string_test.cpp