Remove __ASSUME_EVENTFD2, move eventfd to syscalls.list.

Given current Linux kernel version requirements, we can assume the
presence of the eventfd2 syscall.  This means that __ASSUME_EVENTFD2
can be removed, and a syscalls.list entry suffices for eventfd instead
of needing a .c file.  This patch implements those changes.

Tested for x86_64 and x86 (not that that means much, given the lack of
testsuite coverage for eventfd).

	* sysdeps/unix/sysv/linux/kernel-features.h (__ASSUME_EVENTFD2):
	Remove macro.
	* sysdeps/unix/sysv/linux/eventfd.c: Remove file.
	* sysdeps/unix/sysv/linux/syscalls.list (eventfd): New syscall
	entry.