Note that this includes Nick's fix from edf64cd235 which
landed upstream a bit differently as r13-1566-g9ed57796235abc in GCC.
This pulls in libbacktrace as of r14-9404-gc775a030af9cad in GCC trunk.
Note that I have dropped a top-level Darwin change from r14-4825-g6a6d3817afa02b
which would've required an autoreconf, as it should be handled separately.
Approved-By: Tom Tromey <tom@tromey.com>
