[PATCH] Sync libbacktrace from gcc [PR31327]

[Tom Tromey]

>>>>> "Sam" == Sam James <sam@gentoo.org> writes:

Sam> Note that this includes Nick's fix from edf64cd235f5ecb3725e7cf1ff83bbdb6dd53340 which
Sam> landed upstream a bit differently as r13-1566-g9ed57796235abc in GCC.

Sam> This pulls in libbacktrace as of r14-9404-gc775a030af9cad in GCC trunk.

Thank you.  If this makes it identical to gcc then I think it's fine.
If not perhaps posting the diff would be helpful.

Approved-By: Tom Tromey <tom@tromey.com>

Tom