Remove __ASSUME_O_CLOEXEC / O_CLOEXEC conditionals in sysdeps/unix/sysv/linux/.

Remove __ASSUME_O_CLOEXEC / O_CLOEXEC conditionals in sysdeps/unix/sysv/linux/.

This patch removes conditionals on __ASSUME_O_CLOEXEC, and on
O_CLOEXEC being defined, in sysdeps/unix/sysv/linux/, now that
O_CLOEXEC support can be unconditionally assumed.

The patch is conservative in what it changes and further followup
cleanups may be possible.  It may be possible to remove dl-opendir.c,
but the patch does not do so, just removing a redundant undefine and
redefine of __ASSUME_O_CLOEXEC.  Also, __ASSUME_O_CLOEXEC is defined
unconditionally for Hurd as well as Linux.  Thus, if we decide that
O_CLOEXEC support is a required feature of any glibc port, we could
remove __ASSUME_O_CLOEXEC and all conditionals on it throughout glibc,
rather than just cleaning up sysdeps/unix/sysv/linux/.

Tested x86_64 that the disassembly of installed shared libraries is
unchanged by this patch.

	* sysdeps/unix/sysv/linux/dl-opendir.c (__ASSUME_O_CLOEXEC): Do
	not undefine and redefine.
	* sysdeps/unix/sysv/linux/getsysstats.c (__get_nprocs)
	[O_CLOEXEC]: Make code unconditional.
	(__get_nprocs) [!O_CLOEXEC]: Remove conditional code.
	* sysdeps/unix/sysv/linux/shm_open.c: Do not include
	<kernel-features.h>.
	[O_CLOEXEC && !__ASSUME_O_CLOEXEC] (have_o_cloexec): Remove
	conditional variable definition.
	(shm_open) [O_CLOEXEC]: Make code unconditional.
	(shm_open) [!O_CLOEXEC || !__ASSUME_O_CLOEXEC]: Remove conditional
	code.