Define CLOCKS_PER_SEC type to the type clock_t

C99 specifies that CLOCKS_PER_SEC is an expression with the type clock_t.
This patch adds a generic <bits/time2.h> to define CLOCKS_PER_SEC and
provides the Linux/x86-64 version of <bits/time2.h> to support x32.

	[BZ #17797]
	* bits/time.h (CLOCKS_PER_SEC): Changed to ((clock_t) 1000000).
	* sysdeps/unix/sysv/linux/bits/time.h (CLOCKS_PER_SEC): Likewise.
	* sysdeps/unix/sysv/linux/clock.c (clock): _Static_assert
	CLOCKS_PER_SEC == 1000000.
	* time/clocktest.c (main): Replace %ld with %jd and cast to
	intmax_t.