Extend x32 check by also checking for __x86_64__.

Currently, the configure script checks for x32 by testing just the
__ILP32__ define.  However, on "plain" i386, __ILP32__ can also be
defined, for example by clang 3.5.0 and higher.  (That gcc does not
define it there, is another issue, but not for this tracker.)

Therefore, extend the check by also checking for __x86_64__, which will
also be defined for x32.

BUG=887

Change-Id: I90ac1d6843caff0416e1dd360c0be3dbaa85c2ae
Merge "Extend x32 check by also checking for __x86_64__."