x86: don't open-code REG_{SP,FP}

Since we have the #define-s, we should also use them.
diff --git a/gas/config/tc-i386.c b/gas/config/tc-i386.c
index 532f8f5..4961c2a 100644
--- a/gas/config/tc-i386.c
+++ b/gas/config/tc-i386.c
@@ -3285,8 +3285,8 @@
 #endif
       x86_cie_data_alignment = -8;
 #if defined (OBJ_ELF) || defined (OBJ_MAYBE_ELF)
-      x86_sframe_cfa_sp_reg = 7;
-      x86_sframe_cfa_fp_reg = 6;
+      x86_sframe_cfa_sp_reg = REG_SP;
+      x86_sframe_cfa_fp_reg = REG_FP;
 #endif
     }
   else