LU-2136 tests: build posix test suite on i686 client

LU-2136 tests: build posix test suite on i686 client

This patch improves prep() in lustre/tests/posix/posix.cfg
to support building posix test suite on i686 client.

Test-Parameters: envdefinitions=SLOW=yes \
clientdistro=el6 serverdistro=el6 clientarch=i686 \
serverarch=x86_64 testlist=posix

Signed-off-by: Jian Yu <jian.yu@intel.com>
Change-Id: I7b2fdc5552f0d2eb5cc7ef38b1192ec5a4fdefcf
Reviewed-on: http://review.whamcloud.com/5831
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Minh Diep <minh.diep@intel.com>
Reviewed-by: Wei Liu <wei3.liu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>