toolchain: Add toolchain compiler warning macro indirection

[thughes]

In order to support new toolchains that are not compatible with gcc/clang (e.g., IAR), we need to add a level of indirection for the name of warnings.

Depends on #84063

[tejlmand]

Thanks for opening this.

I think it's very good that we have a look at this and clean up / generalize the usage so that it becomes cleaner and easier to improve toolchain support.

I looked up some of the warnings to see where they are actually being used in code.
Some warnings are disabled in production code, such as nonnull for vector table relocation.
Other warnings are disabled for test-purposes only.
I marked some of then in comments.

I think it's important that we keep the test-only warning disabling separated from the production code, perhaps by creating a dedicated include header which can extend the normal toolchain header(s).

This will achieve two purposes

• Make it clear which warnings are disabled for test purposes (more freedom), and which are disabled in production code (should be better reasoned)
• Minimize the risk of misuse warning disabling macros, by only having those already used in production code generally available. Rests are only available for test code.

[thughes]

See what you think of this version. I moved the test-specific warnings into a header that I think is only accessible to tests: subsys/testsuite/include/zephyr/test_toolchain.h. It's kind of annoying that this file can't just be named subsys/testsuite/include/zephyr/toolchain.h, but it looks like that would conflict with include/zephyr/toolchain.h. Open to other suggestions if you have any.

We could potentially include test_toolchain.h in ztest.h so people wouldn't have to manually include test_toolchain.h for their tests. Not sure if it's better to force them to explicitly acknowledge they're using it or not.

Here's the PR that shows what it looks like when you use it: #84065