arch/x86: prescott is an i686

let CT_ARCH_ARCH=prescott result in a i686 tuple prefix

Signed-off-by: Titus von Boxberg <titus@v9g.de>
(transplanted from d063b1ae8c1b30c7006b110d0f9045e6d7c34f13)

reporter_name="Ioannis E. Venetis"

reporter_url="http://sourceware.org/ml/crossgcc/2011-05/msg00005.html"

reporter_comment=""