Another GLIBC build error with GCC6

[Roland McGrath]

> It looks like it works for any value where the most significant bit is
> set to 1.  Otherwise it gives a different value.  I don't know if we can
> count on that bit always being set or not.

DT_EXTRATAGIDX will only ever be used with the values near DT_FILTER.  To
follow the other existing models, elf.h should define DT_EXTRARNGLO (same
value as DT_AUXILIARY) and DT_EXTRARNGHI (same value as DT_FILTER) and:
#define DT_EXTRATAGIDX(tag) (DT_EXTRARNGHI - (tag)) /* Reverse order! */

DT_VERSIONRNGLO and DT_VERSIONRNGHI are missing too, though
DT_VERSIONTAGIDX is already defined using the same value that
DT_VERSIONRNGHI should have.