Fix bitcode problem exporting with xcodebuild.

[ahamilt]

Exporting archive using xcodebuild -exportOptionsPlist is
failing with the 1.3 release of Multi-OS Engine. The export
fails with an error like this:

Error Domain=IDEDistributionErrorDomain Code=14 "No applicable devices
found." UserInfo={NSLocalizedDescription=No applicable devices found.}

More details in IDEDistribution.standard.log:

"Failed to verify bitcode in MOE.framework/MOE:\nerror: Bundle only
contains bitcode-marker
/var/folders/vb/zpw1mfn94yj6rq6s7byp403r0000gn/T/IDEDistributionThinningStep.qN8/Payload/testfoo.app/Frameworks/MOE.framework/MOE
(armv7)\n\n";

The root problem is that the following file contains a bitcode marker
section, which was not present when using Multi-OS engine 1.2.5:

build/moe/xcodebuild/Release-iphoneos/testfoo.app/Frameworks/MOE.framework/MOE

You can see the section by running the following command:

xcrun size -x -m -l MOE

The difference between 1.2.5 and 1.3 is that 1.2.5 was embedding
the MOE.framework using the xcode project configuration, which
copied the framework with STRIP_BITCODE_FROM_COPIED_FILES set to
the default YES. This stripped out the bitcode marker section
and segment. However 1.3 is copying the framework using a build
script, which doesn't strip out the section.

This commit adds a call to strip out the bitcode from the framework
using the 'bitcode_strip' tool.