sigaltstack.2: ATTRIBUTES: Note function that is thread-safe

The marking matches glibc marking.
The marking of functions in glibc is:
- sigaltstack: MT-Safe

Signed-off-by: Zeng Linggang <zenglg.jy@cn.fujitsu.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/sigaltstack.2

@@ -155,6 +155,19 @@ was less than
 An attempt was made to change the alternate signal stack while
 it was active (i.e., the process was already executing
 on the current alternate signal stack).
+.SH ATTRIBUTES
+For an explanation of the terms used in this section, see
+.BR attributes (7).
+.TS
+allbox;
+lb lb lb
+l l l.
+Interface	Attribute	Value
+T{
+.BR sigaltstack ()
+T}	Thread safety	MT-Safe
+.TE
+
 .SH CONFORMING TO
 SUSv2, SVr4, POSIX.1-2001.
 .SH NOTES