Use embedded DWARF in the iOS apps except for in release build

Running dsymutil takes much too long to be bearable during
development. But when building for actual release we do want a
separate dSYM of course. (Of course none of the current iOS apps in
the source are intended to be actually "released". But add this logic
just for completeness.)

Change-Id: Ibb5037d6926e969a891269d6c9d86232bc01cb3c