Enable stack traces for all executables in all builds

[senderista]

This change significantly simplifies the CMake boilerplate for enabling stack traces on an executable. Now you just need to add this to the CMakeLists.txt for executable foo:

if(ENABLE_STACKTRACE)
  target_link_libraries(foo PRIVATE gaia_stack_trace)
endif()

I've also enabled stack traces by default in release builds (previously they were enabled only in debug builds). I haven't seen any evidence that this will incur a measurable performance penalty, and I think having usable stack traces from crashes in production will be well worth it. But if anyone does want to measure the performance penalty, they can just pass -DENABLE_STACKTRACE=OFF on their cmake command line (don't forget to delete CMakeCache.txt!).

Note that stack traces have been enabled not only on the SDK executables (gaia_db_server, gaiac, gaiat), but also on all test executables (at least those configured via add_gtest()). This should make debugging tests considerably easier, based on my experience.

PS: I have not verified that this works on ARM builds. I'd appreciate it if someone can verify this (there may be additional compiler/linker flags required).