Replace --isa-* options with --cpu option

Accepts arc600, arc700, EM and HS values. arc600 and EM cannot be used with
uClibc tool chain.

Signed-off-by: Anton Kolesov <anton.kolesov@synopsys.com>
(cherry picked from commit 17e4ecbfaa16209e72170f80bdb69c5df93f8171)