new avx unit test failing on 6 cdash builds

Failing in 6 different CDash builds, all release builds looks like: http://dynamorio.org/CDash/testSummary.php?project=1&name=unit_tests&date=2019-03-15

E.g.: http://dynamorio.org/CDash/testDetails.php?test=642583&build=41745

EXPECT failed at /work/dr/nightly/src/core/arch/x86_code.c:596 in test unit_test_get_ymm_caller_saved: memcmp(ref_buffer, get_buffer, proc_num_simd_registers() * MCXT_SIMD_SLOT_SIZE)