target-i386: Make TCG feature filtering more readable

Instead of an #ifdef in the middle of the code, just set
TCG_EXT2_FEATURES to a different value depending on TARGET_X86_64.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
(cherry picked from commit a42d9938)

Conflicts:
	target-i386/cpu.c

*removed dependency on 77549a78

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>