On platforms that support it, enable using ``<execinfo.h>`` to get
backtrace(3) to dump a backtrace on assertion failure. This should make
debugging things like WebContent crashes in Lagom much easier.
Just because we may compile serenity with or without NDEBUG doesn't
mean that consuming projects or Ports will share the setting.
Always define the custom assertion function so that we don't have to
keep the same debug settings between all projects.
Until now, VERIFY() failures would just cause a __builtin_trap() in
release builds, which made them a bit too harsh. This commit adds an
out-of-line helper function that prints the error before trapping.
