Hi Guix!

Since the GCC update, dovecot doesn't build on aarch64 because the
test_backtrace() tests fail :-/.  What happens is the backtrace()
function works but only contains addresses and no real symbol names,
which the tests expect.

I was going to propose disabling them, but doing some searching I found
that debian fixed this by using the -rdynamic linker flag [0], and
indeed, looking at GCC's documentation, they suggest using this flag for
symbols to work [1].

I'm not sure why it works OK on other platforms, this flag might be on
by default there. 

OK to push?

[0]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=962630
[1]: https://www.gnu.org/software/libc/manual/html_node/Backtraces.html