Consolidate Linux sendto implementation

Consolidate Linux sendto implementation

This patch consolidates the sendto Linux syscall implementation on
sysdeps/unix/sysv/linux/sendto.c.  The changes are:

   1. Define __ASSUME_SENDTO_SYSCALL by default.
   2. Undef it for architectures that do not support __NR_sendto.

Checked on i686-linux-gnu, x86_64-linux-gnu, x86_64-linux-gnux32,
aarch64-linux-gnu, arm-linux-gnueabihf, and powerpc64le-linux-gnu.

	* sysdeps/unix/sysv/linux/alpha/syscalls.list: Remove sendto from
	auto-generation list.
	* sysdeps/unix/sysv/linux/arm/syscalls.list: Likewise.
	* sysdeps/unix/sysv/linux/generic/syscalls.list: Likewise.
	* sysdeps/unix/sysv/linux/hppa/syscalls.list: Likewise.
	* sysdeps/unix/sysv/linux/ia64/syscalls.list: Likewise.
	* sysdeps/unix/sysv/linux/mips/syscalls.list: Likewise.
	* sysdeps/unix/sysv/linux/sparc/sparc64/syscalls.list: Likewise.
	* sysdeps/unix/sysv/linux/x86_64/syscalls.list: Likewise.
	* sysdeps/unix/sysv/linux/kernel-features.h
	(__ASSUME_SENDTO_SYSCALL): Define by default.
	* sysdeps/unix/sysv/linux/i386/kernel-features.h
	(__ASSUME_SENDTO_SYSCALL): Undef it is kernel does not support
	__NR_sendto.
	* sysdeps/unix/sysv/linux/m68k/kernel-features.h
	(__ASSUME_SENDTO_SYSCALL): Likewise.
	* sysdeps/unix/sysv/linux/s390/kernel-features.h
	(__ASSUME_SENDTO_SYSCALL): Likewise.
	* sysdeps/unix/sysv/linux/microblaze/kernel-features.h
	(__ASSUME_SENDTO_SYSCALL): Remove definition.
	* sysdeps/unix/sysv/linux/powerpc/kernel-features.h
	(__ASSUME_SENDTO_SYSCALL): Likewise.
	* sysdeps/unix/sysv/linux/sh/kernel-features.h:
	(__ASSUME_SENDTO_SYSCALL): Likewise.
	* sysdeps/unix/sysv/linux/sendto.c: Simplify includes.