This patch consolidates the close Linux syscall generation on
sysdeps/unix/sysv/linux/close.c.
Checked on i686-linux-gnu, x86_64-linux-gnu, x86_64-linux-gnux32,
arch64-linux-gnu, arm-linux-gnueabihf, and powerpc64le-linux-gnu.
* nptl/Makefile (CFLAGS-close.c): New flag.
* sysdeps/unix/sysv/linux/close.c: New file.
