X87: disable the regress-undefined-nan test case for x87.

  The reason is when native_context_specialization flag is ture, X87 turbofan
  will hit the known issue that X87 will change a sNaN to qNaN by default. And
  then it will fail when bit-comparing the source (sNaN) and the result (qNaN).

BUG=

Review URL: https://codereview.chromium.org/1414733004

Cr-Commit-Position: refs/heads/master@{#31530}

test/mjsunit/mjsunit.status

@@ -576,6 +576,12 @@
   'mirror-script': [PASS, NO_VARIANTS],
 }],  # 'arch == mips'
 
+##############################################################################
+['arch == x87', {
+  # Turbofan will hit the known issue that x87 changes sNaN to qNaN by default.
+  'regress-undefined-nan': [SKIP],
+}],  # 'arch == x87'
+
 ##############################################################################
 ['arch == mips64el or arch == mips64', {