longjmp.3: ATTRIBUTES: Note functions that are thread-safe

The markings match glibc markings.
markings of functions in glibc are:
- longjmp:    MT-Safe
- siglongjmp: MT-Safe

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

man3/longjmp.3

@@ -87,6 +87,20 @@ also restores the signal mask that was saved by
 .BR sigsetjmp (3).
 .SH RETURN VALUE
 These functions never return.
+.SH ATTRIBUTES
+For an explanation of the terms used in this section, see
+.BR attributes (7).
+.TS
+allbox;
+lbw23 lb lb
+l l l.
+Interface	Attribute	Value
+T{
+.BR longjmp (),
+.BR siglongjmp ()
+T}	Thread safety	MT-Safe
+.TE
+
 .SH CONFORMING TO
 C89, C99, and POSIX.1-2001 specify
 .BR longjmp ().