[build] Move unwinder to toplevel libgcc

[Rainer Orth]

Paolo Bonzini <bonzini@gnu.org> writes:

> I think Rainer missed the existence of libgcc_tm_file. :)

I didn't, but it lives in gcc/config.gcc, which is contrary to the move
from gcc to libgcc.

	Rainer

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Rainer Orth, Center for Biotechnology, Bielefeld University