In https://github.com/mapbox/mapbox-gl-native/commit/f791eb32b9 we simplified the package
script and intended to remove support for building the SDK without
Bitcode support. The intention was to use the ENABLE_BITCODE project
setting to do this and remove the control of that variable in the
script. However, since we use `xcodebuild build` this only uses
bitcode `marker` by default and causes a failure when an app that uses
the Mapbox SDK is archived.
This adds the `BITCODE_GENERATION_MODE` and `bitcode` value to the
necessary iOS SDK and mbgl and platform targets. If the SDK is compiled
with these flags set, it will be successfully processed in the bitcode
phase in an app archive operation.
This also removes the `BITCODE` flag from the Makefile since the
intention is to not support building the SDK without bitcode support
and the presence of that option might be confusing.
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