libunwind: fix unwinding through libunwind stack frames

[Kha]

This fixes #9591 on x86_64-linux-gnu, and presumably on ARM. Not sure about other platforms, are there platforms that use libunwind but not unwind tables? And should this change come with a test (in test/standalone/c_compiler?)?

[andrewrk]

Thanks for the patch! I'm happy to accept this as is, and will do so now, but if you felt inspired to follow this up with a test case, I would suggest adding a case here: https://github.com/ziglang/zig/blob/master/test/stack_traces.zig and then adding the necessary test harness improvements needed here:

zig/test/tests.zig

Line 576 in 75263e1

pub const StackTracesContext = struct {