disable vp8_sixtap_predict16x16_neon

disable vp8_sixtap_predict16x16_neon

This causes various buffer overflows in the tests:

[ RUN      ] NEON/SixtapPredictTest.TestWithPresetData/0
=================================================================
==22346==ERROR: AddressSanitizer: global-buffer-overflow on address
0x0000012b4a5b at pc 0x000000df0f60 bp 0xffffcf6e64b0 sp 0xffffcf6e64a8
READ of size 8 at 0x0000012b4a5b thread T0
    #0 0xdf0f5c in vp8_sixtap_predict16x16_neon
       vp8/common/arm/neon/sixtappredict_neon.c:1507:13
    #1 0x8819e4 in (anonymous
        namespace)::SixtapPredictTest_TestWithPresetData_Test::TestBody()
       test/predict_test.cc:293:3
    ...

0x0000012b4a5b is located 2 bytes to the right of global variable
'kTestData' defined in '../test/predict_test.cc:237:24' (0x12b48a0) of
size 441

[ RUN      ] NEON/SixtapPredictTest.TestWithRandomData/0
=================================================================
==22338==ERROR: AddressSanitizer: heap-buffer-overflow on address
0xffff8b5321fb at pc 0x000000df0f60 bp 0xfffff7e0cf30 sp 0xfffff7e0cf28
READ of size 8 at 0xffff8b5321fb thread T0
    #0 0xdf0f5c in vp8_sixtap_predict16x16_neon
       vp8/common/arm/neon/sixtappredict_neon.c:1507:13
    #1 0x87d4c0 in (anonymous
       namespace)::PredictTestBase::TestWithRandomData(void (*)(unsigned
       char*, int, int, int, unsigned char*, int))
       test/predict_test.cc:170:9
    ...

0xffff8b5321fb is located 2 bytes to the right of 441-byte region
[0xffff8b532040,0xffff8b5321f9)
allocated by thread T0 here:
    #0 0x5fd4f0 in operator new[](unsigned long) (test_libvpx+0x5fd4f0)
    #1 0x87c2e0 in (anonymous namespace)::PredictTestBase::SetUp()
       test/predict_test.cc:47:12
    #2 0x87d074 in non-virtual thunk to (anonymous
       namespace)::PredictTestBase::SetUp() test/predict_test.cc
    ...

Bug: webm:1795
Change-Id: I32213a381eef91547d00f88acf90f1cf2ec2ea75