icecc: use bb.utils.which also for 'as'

icecc: use bb.utils.which also for 'as'

* it was introduced in
  commit 3a842ec52e7d010767b13bdcb5629ac07b3ee9e7
  Author: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
  Date:   Fri Sep 16 10:55:16 2011 +0400
  Subject: icecc.bbclass: replace with updated version

  without any explanation in which case
    ${ICECC_CC} -print-prog-name=as
  is returning as in current working directory, but will keep old
  behavior just in case

(From OE-Core rev: 6092da20fc3ceb1bc6b4872ad16df565f05723b7)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>