Warn when gettimeofday is called with non-null tzp argument.

Since there are no known uses of gettimeofday's vestigial "get time
zone" feature that are not bugs, add a fortify-style wrapper inline to
sys/time.h that issues a warning whenever gettimeofday is called with
a second argument that is not a compile-time null pointer
constant.

At present this is only possible with GCC; clang does not implement
attribute((warning)).  The wrapper is only activated when __OPTIMIZE__
is defined because it throws false positives when optimization is off,
even though it's an always-inline function.

An oversight in the implementation of __builtin_constant_p causes it
to fail to detect compile-time *pointer* constants unless they are
cast to an integer of a different size.  (Loss of data in this cast is
harmless; the overall expression is still constant if and only if the
original pointer was.)  This is GCC bug 95514.  Thanks to
Kamil Cukrowski <kamilcukrowski@gmail.com> for the workaround.
As a precaution, I added a static assertion to debug/warning-nop.c to
make sure that the cast _is_ casting to an integer of a different
size; this is too unlikely a scenario to be worth checking in the
public header, but if someone ever adds a port where short is the
same size as intptr_t, we'll still catch it.

Also make the public prototype of gettimeofday declare its second
argument with type "void *" unconditionally, consistent with POSIX.

	* time/sys/time.h (__timezone_ptr_t): Delete.
	(gettimeofday): Always declare second argument with type "void *".
	When possible, wrap with a fortify-style inline function that
	detects non-null or non-constant second argument and issues a
	warning.  Improve commentary.
	(settimeofday): Improve commentary.

	* time/gettimeofday.c (gettimeofday): Declare second argument with
	type "void *".
	* debug/warning-nop.c: Include sys/time.h and stdint.h.
	Add static_assert to verify the requirements of the workaround
	for GCC bug 95514.

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