This patch consolidates the write Linux syscall implementation on
sysdeps/unix/sysv/linux/write.c.
Checked on i686-linux-gnu, x86_64-linux-gnu, x86_64-linux-gnux32,
arch64-linux-gnu, arm-linux-gnueabihf, and powerpc64le-linux-gnu.
* include/unistd.h (write): Add hidden proto.
* io/Makefile (CFLAGS-write.c): New rule.
* nptl/Makefile (CFLAGS-write.c): Likewise.
* sysdeps/unix/sysv/linux/write.c: New file.
