Define XTABS to TAB3 on alpha to match Linux 4.16.

This patch makes the alpha bits/termios.h define XTABS to TAB3, so
matching a change made in Linux 4.16 as well as matching other
architectures where the values are already equal.

Tested with build-many-glibcs.py for alpha-linux-gnu.

	* sysdeps/unix/sysv/linux/alpha/bits/termios.h [__USE_MISC]
	(XTABS): Define to TAB3.