rs6000: Replace outdated link to ELFv2 ABI

A user reported that we are still referring to a public review
draft of the ELFv2 ABI specification.  Replace that by a permalink.

2020-04-24  Bill Schmidt  <wschmidt@linux.ibm.com>

	* gcc/doc/extend.texi (PowerPC AltiVec/VSX Built-in Functions):
	Replace outdated link to ELFv2 ABI.

gcc/ChangeLog

+2020-04-23  Bill Schmidt  <wschmidt@linux.ibm.com>
+
+	* gcc/doc/extend.texi (PowerPC AltiVec/VSX Built-in Functions):
+	Replace outdated link to ELFv2 ABI.
+
 2020-04-23  Jakub Jelinek  <jakub@redhat.com>
 
 	PR target/94710

gcc/doc/extend.texi

@@ -17633,7 +17633,7 @@ subject to change without notice.
 
 GCC complies with the OpenPOWER 64-Bit ELF V2 ABI Specification,
 which may be found at
-@uref{http://openpowerfoundation.org/wp-content/uploads/resources/leabi-prd/content/index.html}.
+@uref{https://openpowerfoundation.org/?resource_lib=64-bit-elf-v2-abi-specification-power-architecture}.
 Appendix A of this document lists the vector API interfaces that must be
 provided by compliant compilers.  Programmers should preferentially use
 the interfaces described therein.  However, historically GCC has provided