default CPU target for ix86 based ports

[Matthias Klose <doko@cs.tu-berlin.de>]

Now that the kernel-image packages supports hw emulation of i486
instructions on i386 hardware, I'd like to change the code generation
to default to i486 (not sure if it should be tuned for any other
target, i.e. -mtune=i686).

IIRC the Hurd can be built for i586 only, so it could be used as the
default target CPU as well.

Unsure about the bsd targets.

	Matthias