Update some outdated links about XLC and UNIX specification

Author: Vignesh C
Discussion: https://postgr.es/m/CALDaNm3Dy=dTdx8UCVw=DWbzLzmRUC1dkq45=heOZDUg3U_PtA@mail.gmail.com

src/backend/utils/mb/wchar.c

@@ -571,7 +571,7 @@ pg_utf_mblen(const unsigned char *s)
 /*
  * This is an implementation of wcwidth() and wcswidth() as defined in
  * "The Single UNIX Specification, Version 2, The Open Group, 1997"
- * <http://www.UNIX-systems.org/online.html>
+ * <http://www.unix.org/online.html>
  *
  * Markus Kuhn -- 2001-09-08 -- public domain
  *

src/include/c.h

@@ -101,8 +101,8 @@
  * GCC: https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html
  * GCC: https://gcc.gnu.org/onlinedocs/gcc/Type-Attributes.html
  * Sunpro: https://docs.oracle.com/cd/E18659_01/html/821-1384/gjzke.html
- * XLC: http://www-01.ibm.com/support/knowledgecenter/SSGH2K_11.1.0/com.ibm.xlc111.aix.doc/language_ref/function_attributes.html
- * XLC: http://www-01.ibm.com/support/knowledgecenter/SSGH2K_11.1.0/com.ibm.xlc111.aix.doc/language_ref/type_attrib.html
+ * XLC: https://www.ibm.com/support/knowledgecenter/SSGH2K_13.1.2/com.ibm.xlc131.aix.doc/language_ref/type_attrib.html
+ * XLC: https://www.ibm.com/support/knowledgecenter/SSGH2K_13.1.2/com.ibm.xlc131.aix.doc/language_ref/type_attrib.html
  */
 
 /* only GCC supports the unused attribute */