[build-script] Allow to tune dsymutil parallelism (#34795)
This should enable scaling when using machines with large amount of
RAM.
To better support machines with lower spec, process one binary per
dsymutil invocation (reverting #34149).
Add some (limited) facilities to gather the time taken to execute
dsymutil to better assist in tuning the parameter -- these are printed in
JSON format in the log to allow for easier scraping
```
{ "command": "dsymutil", "start": "2020-11-18T18:10:47" }
{ "command": "dsymutil", "end": "2020-11-18T18:14:45" }
```
Addresses rdar://71018443
