[RS6000] Fix rs6000_adjust_atomic_subword in little-endian mode

[Anton Blanchard]

Fixes gcc/testsuite/gcc.dg/sync-2.c

        * config/rs6000/rs6000.c (rs6000_adjust_atomic_subword): Calculate
        correct shift value in little-endian mode.

diff --git a/gcc/config/rs6000/rs6000.c b/gcc/config/rs6000/rs6000.c
--- a/gcc/config/rs6000/rs6000.c
+++ b/gcc/config/rs6000/rs6000.c
@@ -17314,8 +17314,9 @@ rs6000_adjust_atomic_subword (rtx orig_mem, rtx 
*pshift, rtx *pmask)
   shift = gen_reg_rtx (SImode);
   addr = gen_lowpart (SImode, addr);
   emit_insn (gen_rlwinm (shift, addr, GEN_INT (3), GEN_INT (shift_mask)));
-  shift = expand_simple_binop (SImode, XOR, shift, GEN_INT (shift_mask),
-                              shift, 1, OPTAB_LIB_WIDEN);
+  if (WORDS_BIG_ENDIAN)
+    shift = expand_simple_binop (SImode, XOR, shift, GEN_INT (shift_mask),
+                                shift, 1, OPTAB_LIB_WIDEN);
   *pshift = shift;
 
   /* Mask for insertion.  */