Detect presence of sse3 instructions on x86.
Set VEX_HWCAPS_X86_SSE3 accordingly.
This came about by grepping the source for VEX_HWCAPS_X86_SSE3
and observing that the flag was tested for (in VEX) but not set.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14803 a5019735-40e9-0310-863c-91ae7b9d1cf9