termios: Consolidate Baud Rate Selection definitions (BZ#23783)

This patch consolidates the termios symbolic constants used for baud rates
selection used along with speed_t on its own header.  The Linux generic
implementation values match the kernel UAPI and each architecture with
deviate values have their own implementation (in this case alpha and
powerpc).

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

	[BZ #23783]
	* sysdeps/unix/sysv/linux/Makefile (sysdep_headers): Add
	termios-baud.h.
	* sysdeps/unix/sysv/linux/bits/termios-baud.h: New file.
	* sysdeps/unix/sysv/linux/alpha/bits/termios-baud.h: Likewise.
	* sysdeps/unix/sysv/linux/powerpc/bits/termios-baud.h: Likewise.
	* sysdeps/unix/sysv/linux/sparc/bits/termios-baud.h: Likewise.
	* sysdeps/unix/sysv/linux/alpha/bits/termios.h (B57600, B115200,
	B230400, B460800, B500000, B576000, B921600, B1000000, B1152000,
	B1500000, B2000000, B2500000, B3000000, B3500000, B4000000,
	__MAX_BAUD): Move to termios-baud.h.
	[__USE_MISC] (CBAUD, CBAUDEX): Likewise.
	* sysdeps/unix/sysv/linux/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.