i386: punt all config options that just control -march

You can set these yourself in your CFLAGS, and our list is incomplete
and pointless (we agreed some time ago to stop doing this).

We keep around CONFIG_[3456]86 because it is used to select between
different optimized subdirs of handcoded assembly.

Also take this opportunity to up the default to i686.  If you're using
an older cpu, then you can handle changing this yourself.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>