PPC: Disable gcc generation of fmadd/fmsub in cctests.

Generated code performs distinct floating multiply and add/subtract
operations.  Tests fail when GCC uses fmadd/fmsub to calculate the
expected result since these instructions provide higher accuracy due
to the lack of an intermediate round.

R=machenbach@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1416123007

Cr-Commit-Position: refs/heads/master@{#31918}

test/cctest/cctest.gyp

@@ -279,6 +279,11 @@
             },
           },
         }],
+        ['v8_target_arch=="ppc" or v8_target_arch=="ppc64"', {
+          # disable fmadd/fmsub so that expected results match generated code in
+          # RunFloat64MulAndFloat64Add1 and friends.
+          'cflags': ['-ffp-contract=off'],
+        }],
         ['OS=="aix"', {
           'ldflags': [ '-Wl,-bbigtoc' ],
         }],