Implement jl_rec_backtrace for ASM/SETJMP on FreeBSD

[ararslan]

This removes the message emitted while compiling on FreeBSD that says it isn't supported.

That said, while the code compiles without issue, I'm not certain it's actually correct. It seems jl_rec_backtrace is only used in jlbacktracet, which is listed among the GDB utilities, so perhaps GDB (or LLDB?) could be used to check against the result on another system. I'd just need some guidance for how to do so as I'm generally unfamiliar with this part of the code.

[ararslan]

I've been trying to exercise this code path but I can't quite get there. The closest I've come is

@async begin
    try
        error("bad")
    catch
        ccall(:jlbacktracet, Cvoid, (Any,), current_task())
    end
end

which at least shows jl_rec_backtrace.

[ararslan]

I assume this looks correct enough then 😅