It is a diff mark.
In the abstract (p.1) of N1570:
Changes from the previous draft (N1539) are indicated by ‘‘diff marks’’ in the right margin: deleted text is marked with ‘‘∗’’, new or changed text with ‘‘ ’’.
In the N1539 draft, alignof
was a keyword, appearing on the first row of that table:
N1750 dropped alignof
and replaced it with _Alignof
.
The placement of the "*" does seem slightly odd, but it definitely doesn't refer to the *
operator.