Avoid unnecessarily calling slower functions

[N-R-K]

For example, in strcpy (ignoring the UB overlapping check):

fortify-headers/include/string.h

Lines 139 to 150 in 9aa4490

	size_t __n = strlen(__s) + 1;
	/* trap if pointers are overlapping but not if dst == src.
	* gcc seems to like to generate code that relies on dst == src */
	if ((__d < __s && __d + __n > __s) ||
	(__s < __d && __s + __n > __d))
	__builtin_trap();
	size_t __b = __bos(__d, 0);
	if (__n > __b)
	__builtin_trap();
	return __orig_strcpy(__d, __s);

By the time, __orig_strcpy is called, the length has already been computed and has been established to be in bound. There's no need to pay for the nul-byte search again inside of __orig_strcpy when the faster memcpy can be called instead.

Some of the other str* functions suffer from the same issue.

[jvoisin]

The rationale behind this was that maybe other things were "hooking" strcpy, but you're right, a call to memcpy makes sense.