Config.in.arch: add help and less cryptic names to architecture menu

[Peter: fixup s/big-endian/big endian/ as pointed out by Thomas]
Signed-off-by: Alvaro G. M <alvaro.gamez@hazent.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>