libgo: print reason code if throwing unwind exception fails

Calls to _Unwind_RaiseException and friends *can* return due to bugs in
libgo or memory corruption. When this occurs, print a message to stderr
with the reason code before aborting to aid debugging.

Change-Id: Iadd7bfca40def25fa7fa9208dca52a56dbbe2208
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/261257
Trust: Than McIntosh <thanm@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
1 file changed