Simplify sendmmsg code.

Now we can assume a kernel with sendmmsg support, this patch
simplifies the implementation to be similar to that for accept4:
either using socketcall or the syscall according to whether the
syscall is known to be available, without further fallback
implementations.  The __ASSUME_SENDMMSG macro is kept (now defined
unconditionally), since it's used in resolv/res_send.c.

Tested for x86_64 and x86.

	* sysdeps/unix/sysv/linux/kernel-features.h
	(__ASSUME_SENDMMSG_SYSCALL): Define unconditionally.
	(__ASSUME_SENDMMSG): Likewise.
	(__ASSUME_SENDMMSG_SOCKETCALL): Remove macro.
	* sysdeps/unix/sysv/linux/sendmmsg.c (__sendmmsg): Define using
	sendmmsg syscall if that can be assumed to be present, socketcall
	otherwise, with no fallback for runtime failure.