x86: Use "%v" to emit VEX encoded instructions for AVX targets

x86: Use "%v" to emit VEX encoded instructions for AVX targets

Legacy encodings of SSE instructions incur AVX-SSE domain transition
penalties on some Intel microarchitectures (e.g. Haswell, Broadwell).
Using the VEX forms avoids these penatlies and keeps all instructions
in the VEX decode domain.  Use "%v" sequence to emit the "v" prefix
for opcodes when compiling with -mavx.

No functional changes intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Reviewed-by: Florian Weimer <fweimer@redhat.com>
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>