gdb: amd64 linux coredump support with shadow stack.

gdb: amd64 linux coredump support with shadow stack.

Intel's Control-Flow Enforcement Technology (CET) provides the shadow
stack feature for the x86 architecture.

This commit adds support to write and read the shadow-stack node in
corefiles.  This helps debugging return address violations post-mortem.
The format is synced with the linux kernel commit "x86: Add PTRACE
interface for shadow stack".  As the linux kernel restricts shadow
stack support to 64-bit, apply the fix for amd64 only.

Co-Authored-By: Christina Schimpe <christina.schimpe@intel.com>
Reviewed-By: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Approved-By: Luis Machado <luis.machado@arm.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>
---
The code and testcase are lightly adapted from:

[PATCH v3 5/9] GDB, gdbserver: aarch64-linux: Initial Guarded Control Stack support

https://sourceware.org/pipermail/gdb-patches/2025-June/218892.html