-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

So, in order to avoid ending up with a flexible-array member in the
middle of multiple other structs, we use the `__struct_group()`
helper to create a new tagged `struct glink_msg_hdr`. This structure
groups together all the members of the flexible `struct glink_msg`
except the flexible array.

As a result, the array is effectively separated from the rest of the
members without modifying the memory layout of the flexible structure.
We then change the type of the middle struct members currently causing
trouble from `struct glink_msg` to `struct glink_msg_hdr`.

We also want to ensure that when new members need to be added to the
flexible structure, they are always included within the newly created
tagged struct. For this, we use `static_assert()`. This ensures that the
memory layout for both the flexible structure and the new tagged struct
is the same after any changes.

This approach avoids having to implement `struct glink_msg_hdr` as a
completely separate structure, thus preventing having to maintain two
independent but basically identical structures, closing the door to
potential bugs in the future.

We also use `container_of()` whenever we need to retrieve a pointer to
the flexible structure, through which we can access the flexible-array
member, if necessary.

Additionally, we use the `DEFINE_RAW_FLEX()` helper for an on-stack
definition of a flexible structure where the size for the flexible-array
member is known at compile-time.

So, with these changes, fix the following warnings:
drivers/rpmsg/qcom_glink_native.c:51:26: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/rpmsg/qcom_glink_native.c:459:34: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/rpmsg/qcom_glink_native.c:846:34: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/rpmsg/qcom_glink_native.c:968:34: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/rpmsg/qcom_glink_native.c:1380:34: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Link: https://lore.kernel.org/r/ZrOQa2gew5yadyt3@cuteSigned-off-by: Bjorn Andersson <andersson@kernel.org>