Remove __ASSUME_PPOLL.

Remove __ASSUME_PPOLL.

With current kernel version requirements, the ppoll Linux syscall can
be assumed to be present on all architectures; this patch removes the
__ASSUME_PPOLL macro and conditionals on it and on whether __NR_ppoll
is defined.  (Note that the same can't yet be done for pselect,
because MicroBlaze only wired that up in the syscall table in 3.15.)

Tested for x86_64 and x86 that installed stripped shared libraries are
unchanged by the patch.

	* sysdeps/unix/sysv/linux/kernel-features.h (__ASSUME_PPOLL):
	Remove macro.
	* sysdeps/unix/sysv/linux/ppoll.c: Do not include
	<kernel-features.h>.
	[__NR_ppoll]: Make code unconditional.
	[!__ASSUME_PPOLL]: Remove conditional code.