Test cases for 128-bit --partial-loads-ok=no|yes on x86 need a prereq on sse.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13514 a5019735-40e9-0310-863c-91ae7b9d1cf9
Support mmxext (integer sse) subset on i386 (athlon). Bug #323713

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

Detects mmxext subset from cpuid information (and enables it
when full sse1 is found). Also fixes the prereq of
none/tests/x86/insn_mmxext.vgtest so that it also runs when
full sse1 (and not just the mmxext subset) is found.
It already passed on such configurations. With the VEX patch
(r2745) it also passes with just the mmxext subset.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13515 a5019735-40e9-0310-863c-91ae7b9d1cf9