Force Bitcode in make ipackage

[1ec5]

make ipackage now performs a Bitcode-enabled archive build for devices, making it possible to archive a Bitcode-enabled application that links against the Mapbox iOS SDK.

@incanus, can you verify that the resulting static library contains all the expected symbols? I did notice some additional missing symbol warnings in the CreateUniversalBinary step, such as the one below, but I think those are the same things we try to suppress when calling libtool with the -no_warning_for_no_symbols flag:

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool: for architecture: x86_64 file: /Users/mxn/hub/mapbox-gl-native/build/mbgl.build/Release-iphonesimulator/core.build/Objects-normal/x86_64/libmbgl-core.a(camera.o) has no symbols

/ref #2332
/cc @mikemorris

[mikemorris]

These are the same warnings I'm seeing in Xcode when building a CocoaPods project with libMapbox.a copied over from #2584 (comment)

[incanus]

Looking good here.

[incanus]

Merging ahead of Travis running here, since this only affects the iOS package script and we don't build iOS on Travis.