Cross-reference abort2(2) from a few man pages

I didn't know abort2 existed until it was mentioned on a mailing list.
Mention it in related pages so others can find it easily.

MFC after:	3 days
Sponsored by:	Dell EMC

lib/libc/stdlib/abort.3

@@ -64,6 +64,7 @@ The
 function
 never returns.
 .Sh SEE ALSO
+.Xr abort2 2 ,
 .Xr sigaction 2 ,
 .Xr exit 3
 .Sh STANDARDS

lib/libc/stdlib/exit.3

@@ -116,6 +116,7 @@ functions
 never return.
 .Sh SEE ALSO
 .Xr _exit 2 ,
+.Xr abort2 2 ,
 .Xr wait 2 ,
 .Xr at_quick_exit 3 ,
 .Xr atexit 3 ,

share/man/man3/assert.3

@@ -85,6 +85,7 @@ generates a diagnostic message similar to the following:
 .Pp
 .Dl "Assertion failed: (1 == 0), function main, file assertion.c, line 100."
 .Sh SEE ALSO
+.Xr abort2 2 ,
 .Xr abort 3
 .Sh STANDARDS
 The