fix #3781

this was due to println blocking and switching tasks during macroexpansion.
uses of fl_gc_handle were holding on to stack addresses, which cannot be
done around calls to arbitrary julia code. I believe the only place this
was happening was fl_map1, which calls flisp code that might call julia
code. other uses of fl_gc_handle only surround flisp object allocations,
which is ok.
Merge branch 'master' of github.com:JuliaLang/julia