target-i386: tcg: Check right CPUID bits for clflushopt/pcommit

Detect the clflushopt and pcommit instructions and check their
corresponding feature flags, instead of checking CPUID_SSE and
CPUID_CLFLUSH.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
