use __i386__ macro to detect i386 arch
use __i386__ macro to detect i386 arch

* clang does not define `i386' macro with `-Wc++11-extensions'
  and `-arch i386' options.
* and the __i386__, __arm__, __mips__ macros are more commonly
  used and supported by both GCC and clang. so change it.

Change-Id: Iebbd2def438893c864f87d2d28e51a905b9a7cf9
Signed-off-by: Kefu Chai <ef1142690fdf7c5d47bbd7e2a7a410478308cc21@mozy.com>
Reviewed-on: https://gerrit.dechocorp.com/41493
Reviewed-by: Jenkins <49695b0f9c673fd73dab0ff3f84b5ced9302cc6a@mozy.com>
Reviewed-by: John Bowers <d7b9308ed434571742742576b893415f6b289a2d@vmware.com>
Reviewed-by: Kefu Chai <a4b264ee037c625052534d60505bfd0db4776ac4@vmware.com>

adfin/mordor,adfin/mordor,mtanski/mordor,mozy/mordor,adfin/mordor,mozy/mordor,mtanski/mordor,mtanski/mordor,mozy/mordor