Add -Wl,--export-dynamic to standard LDFLAGS if supported

From the ld man page:

  When creating a dynamically linked executable, using the -E option or
  the --export-dynamic option causes the linker to add all symbols to
  the dynamic symbol table.  The dynamic symbol table is the set of
  symbols which are visible from dynamic objects at run time.

This should allow the backtrace(3) to fully resolve the symbols when
creating backtrace on an assertion failure.

(cherry picked from commit b05e20c9)
68 jobs for !6255 with ondrej-use--Wl,--export-dynamic-v9_18 in 30 minutes and 51 seconds (queued for 3 seconds)
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