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="Yann E. MORIN"

reporter_url="http://ymorin.is-a-geek.org/projects/crosstool"

reporter_comment="Experimental toolchain for ARM cortex-m3 in Thumb-only, noMMU, and newlib."