Support mmxext (integer sse) subset on i386 (athlon).

Some processors like the AMD Athlon "Classic" support mmxext,
a sse1 subset. This subset is not properly detected by VEX.
The subset uses the same encoding as the sse1 instructions.

The subset is described at:
  http://support.amd.com/us/Embedded_TechDocs/22466.pdf
  https://en.wikipedia.org/wiki/3DNow!#3DNow.21_extensions

This introduces a new VEX_HWCAPS_X86_MMXEXT that sits between
the baseline (0) and VEX_HWCAPS_X86_SSE1. There is also a new
x86g_dirtyhelper_CPUID_mmxext to mimics a Athlon "Classic"
(Model 2, K75 "Pluto/Orion").

Groups all mmxext instructions together in one block.

git-svn-id: svn://svn.valgrind.org/vex/trunk@2745 8f6e269a-dfd6-0310-a8e1-e2731360e62c