dsym support on MacOS

[ulfjack]

We support fission on Linux, we should also support dsym on MacOS. I was hoping that we could use the same code path as fission, but it looks like it's a bit more tricky.

[ulfjack]

Someone was working on this internally, but he doesn't seem to be on github, so I can't easily assign it to him.

[dmaclach]

try adding --objc_generate_debug_symbols=true to your build options

[RNabel]

There is a flag --apple_generate_dsym -- since this flag exists, does it work, and if so, why is this issue still open?

[sergiocampama]

I believe this is for blaze dsym symbols.--apple_generate_dsym is for Apple builds using bazel, not for building bazel itself.