devel/freebsd-gcc6: fix build on powerpc64 elfv2

Use the same patch that was committed for lang/gcc9 to fix build.

PR:		243933
Reported by:	jhb (maintainer)

devel/freebsd-gcc6/files/patch-clang-vec_step

@@ -0,0 +1,21 @@
+https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=239266 reports how
+clang unfortunately poisons user namespace by default (without any
+special options).
+
+Until that changes (or GCC changes) we need to avoid using vec_step
+as a variable.
+
+--- UTC
+Index: gcc/tree-vect-loop.c
+===================================================================
+--- gcc/tree-vect-loop.c	(revision 273856)
++++ gcc/tree-vect-loop.c	(working copy)
+@@ -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.