Add support for vex coding scheme in x86 assembler

This patch adds support to emit VEX prefix which is needed
to emit instructions namely andn, blsmsk, blsr, blsi
on a cpu that has AVX2.

Test: ./test.py --host --64, test-art-host-gtest
Change-Id: I6b4902caf8560e4406c5053b142686ed28ba5404
Signed-off-by: Shalini Salomi Bodapati <shalini.salomi.bodapati@intel.com>