ppc64: Use assembly files only if __VSX__ is defined.

ChangeLog

 2025-04-02  Niels Möller  <nisse@lysator.liu.se>
+	Cherry-picked from branch nettle-3.10-fixes:
+	* configure.ac: For ppc64, check if __VSX__ is defined, and
+	disable use of assembly if it isn't.
+
 	* .gitlab-ci.yml: Fix native/fat to actually run make
 	check-fat-override. Add an EXTRA_CHECK_TARGET variable also for
 	the .cross-build template, use it for check-fat-override where

configure.ac

@@ -367,6 +367,16 @@ case "$host_cpu" in
       ]], [[]])], [], [
 	ELFV2_ABI=yes
       ])
+      if test "$enable_assembler" = yes ; then
+	AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
+#if defined(__VSX__)
+#error VSX available
+#endif
+      ]], [[]])], [
+	  AC_MSG_WARN([VSX instructions (Power7 or later) not available, disabling use of assembly.])
+	  enable_assembler=no
+      ], [])
+      fi
     fi
     ;;
   aarch64*)