correct SSSE3 detection; fixes #300

SSSE3 is indicated by bit 9 of ECX, not bit 0, which indicates the
presence of SSE3.

There are very few CPUs in use affected by this bug; SSE3 was part of
the Prescott new instructions, introduced in the later Pentium 4 chips,
whereas SSSE3 was introduced in Intel's Core 2 and AMD's Bulldozer. This
leaves a few Pentium 4 and Athlon 64 models that will potentially run an
illegal pshufb or pblendw.

c/blake3_dispatch.c

@@ -101,7 +101,7 @@ static
     if (*edx & (1UL << 26))
       features |= SSE2;
 #endif
-    if (*ecx & (1UL << 0))
+    if (*ecx & (1UL << 9))
       features |= SSSE3;
     if (*ecx & (1UL << 19))
       features |= SSE41;