Sync libbacktrace from gcc [PR31327]

Note that this includes Nick's fix from edf64cd2 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>