cmd/link/internal/loadpe: fix .xdata unwind info parsing
Unwind info in .xdata was being parsed incorrectly, causing targetOff to
be incorrect and miss finding data in .xdata that it should have found.
This causes a linker issue when using the MinGW MSVCRT compiler.
Contains several fixes based on the exception handling docs: the offset
used to get the number of unwind codes, the calculation of the target
offset based on the dynamic size of the unwind data, and the
UNW_FLAG_CHAININFO flag's value.
Fixes #64200
Change-Id: I6483d921b2bf8a2512a95223bf3c8ce8bc63dc4a
Reviewed-on: https://go-review.googlesource.com/c/go/+/544415
TryBot-Result: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com> Reviewed-by: Than McIntosh <thanm@google.com> Reviewed-by: Michael Knyszek <mknyszek@google.com>