Document the flag needed for function names in stack backtraces.

[martin-cs]

Glibc and Apple's libc both provide support for stack introspection.
This is used by the invariant code to generate a stack backtrace
on invariant failure. By default GCC strips out the function names
so an additional LINKFLAG is needed to get function names in the
backtrace. This was hidden^Wdocumented in util/invariant.cpp, this
patch adds it to config.inc so it can be more widely understood
and used.

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 57e8cc6).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/92861207

[kroening]

I do find the names useful; please add to Travis/codebuild config.