PPC: Fix NaN Canonicalization.

Use fsub rather than fadd to avoid stripping the sign from minus zero.

Fixes mjsunit/result-table-min and mjsunit/result-table-max test failures.

R=michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1084583003

Cr-Commit-Position: refs/heads/master@{#27794}

src/ppc/macro-assembler-ppc.cc

@@ -602,7 +602,7 @@ MemOperand MacroAssembler::SafepointRegistersAndDoublesSlot(Register reg) {
 void MacroAssembler::CanonicalizeNaN(const DoubleRegister dst,
                                      const DoubleRegister src) {
   // Turn potential sNaN into qNaN.
-  fadd(dst, src, kDoubleRegZero);
+  fsub(dst, src, kDoubleRegZero);
 }