Consolidate Linux writev implementation

This patch consolidates the writev Linux syscall implementation on
sysdeps/unix/sysv/linux/writev.c.

Checked on i686-linux-gnu, x86_64-linux-gnu, x86_64-linux-gnux32,
arch64-linux-gnu, arm-linux-gnueabihf, and powerpc64le-linux-gnu.

	* sysdeps/unix/sysv/linux/writev.c: New file.