CBLT-239:  for centos-x86, set ARCHP=i386, leave ARCH as is

Change-Id: I2f43fa4838fcd033ca2f24089a3187af6dcbde83
Reviewed-on: http://review.couchbase.org/35278
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>