termios: Consolidate tcflow symbolic constants

This patch consolidates the termios symbolic constants for use with tcflow
in its own header.  The Linux generic implementation values match the
kernel UAPI and each architecture with deviate values have their own
implementation (currently only mips).

No semantic change is expected, checked on a build against x86_64-linux-gnu,
alpha-linux-gnu, mips64-linux-gnu, and sparc64-linux-gnu.

	* sysdeps/unix/sysv/linux/Makefile (sysdep_headers): Add
	termios-tcflow.h.
	* sysdeps/unix/sysv/linux/bits/termios-tcflow.h: New file.
	* sysdeps/unix/sysv/linux/mips/bits/termios-tcflow.h: Likewise.
	* sysdeps/unix/sysv/linux/bits/termios.h (TCSANOW, TCSADRAIN,
	TCSAFLUSH): Move to termios-tcflow.h.
	* sysdeps/unix/sysv/linux/alpha/bits/termios.h: Likewise.
	* sysdeps/unix/sysv/linux/mips/bits/termios.h: Likewise.
	* sysdeps/unix/sysv/linux/powerpc/bits/termios.h: Likewise.
	* sysdeps/unix/sysv/linux/sparc/bits/termios.h: Likewise.