arch: sparc: Add leon3 cpu type and remove sparc{s,h}fleon{,v8}

There is support for -mcpu=leon3 from gcc 4.8.3. Use this for LEON systems
instead of the non-mainline targets sparcsfleon, sparchfleon, sparcsfleonv8, and
sparchfleonv8.

[Thomas: add Config.in.legacy handling for the removed options.]

Signed-off-by: Andreas Larsson <andreas@gaisler.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>