Detect atomic operations support on ppc64le

Current configure detects only stdatomic operations. Fix it to detect
also ISC atomics with assembly.

aclocal.m4

@@ -1,6 +1,6 @@
-# generated automatically by aclocal 1.16.1 -*- Autoconf -*-
+# generated automatically by aclocal 1.16.2 -*- Autoconf -*-
 
-# Copyright (C) 1996-2018 Free Software Foundation, Inc.
+# Copyright (C) 1996-2020 Free Software Foundation, Inc.
 
 # This file is free software; the Free Software Foundation
 # gives unlimited permission to copy and/or distribute it,

configure

@@ -21847,7 +21847,7 @@ _ACEOF
 	alpha*-*)
 		arch=alpha
 	;;
-	powerpc-*|powerpc64-*)
+	powerpc-*|powerpc64*-*|powerpc64el-*)
 		arch=powerpc
 	;;
 	mips-*|mipsel-*|mips64-*|mips64el-*)

configure.ac

@@ -4123,7 +4123,7 @@ if test "yes" = "$use_atomic"; then
 	alpha*-*)
 		arch=alpha
 	;;
-	powerpc-*|powerpc64-*)
+	powerpc-*|powerpc64*-*|powerpc64el-*)
 		arch=powerpc
 	;;
 	mips-*|mipsel-*|mips64-*|mips64el-*)