[PATCH] Provide unwind info for x86, x86-64 (and s390{,x}) C++ thunks (PR target/43498)

[Jan Hubicka]

> Hi!
> 
> We don't emit unwind info on these targets for C++ thunks, while we do so e.g.
> for ppc/ppc64/ia64/sparc*.
> I've finally thought about just emitting .cfi_startproc/.cfi_endproc when
> -fdwarf2-cfi-asm, but as the thunks are emitted before .eh_frame with
> -fno-dwarf2-cfi-asm, it actually isn't any harder to handle it for both
> ways of emitting unwind info.

Note that original idea with defining unwinding in x86_64 ABI was that for
functions with no stack frame there should be no need for dwarf2 unwind info,
since the unwinders can easilly handle it themselves.  This was meant primarily
to make unwinding through trampolines possible, but thunks falls into this category
too.

Honza