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.
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