Fix bitcode generation for Release builds

[conradev]

The current binary release of Realm for watchOS is unsuitable for submitting to the App Store.

It was compiled with -fembed-bitcode-marker and not -fembed-bitcode, meaning the bitcode section is size 1, or empty:

$ otool -l ./realm-objc-0.95.1/watchos/Realm.framework/Realm | grep -A 5 __bundle
  sectname __bundle
   segname __LLVM
      addr 0x00184000
      size 0x00000001
    offset 1589248
     align 2^0 (1)

This is because BITCODE_GENERATION_MODE is normally set to marker when building, and only set to bitcode when archiving. I assume this is for performance reasons.

This pull request embeds bitcode in all Release builds.

[realm-ci]

Thanks for the PR, @conradev! We'll need you to accept our Contributor License Agreement before we can merge your contributions.

Cheers,
The robots and humans at Realm

[jpsim]

Thanks a lot for this, and for explaining what was wrong. Your thoroughness is appreciated.

If this fix is indeed correct (and it sure looks like it is), I imagine the same should be done for the iOS Dynamic scheme as well as the RealmSwift.xcodeproj xcconfig.