clang on powerpc64 defines vec_step already which collides with the
symbol in gcc, see https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=239266#c23
renaming suggested at https://gcc.gnu.org/legacy-ml/gcc/2019-07/msg00131.html

Index: gcc/tree-vect-loop.c
--- gcc/tree-vect-loop.c.orig
+++ gcc/tree-vect-loop.c
@@ -55,6 +55,8 @@ along with GCC; see the file COPYING3.  If not see
 #include "vec-perm-indices.h"
 #include "tree-eh.h"
 
+#define vec_step vec_step_
+
 /* Loop Vectorization Pass.
 
    This pass tries to vectorize loops.