compiler: Use __typeof_unqual__() for __unqual_scalar_typeof()

The recent changes to get_unaligned() resulted in a new sparse warning:

   net/rds/ib_cm.c:96:35: sparse: sparse: incorrect type in argument 1 (different modifiers) @@     expected void * @@     got restricted __be64 const * @@
   net/rds/ib_cm.c:96:35: sparse:     expected void *
   net/rds/ib_cm.c:96:35: sparse:     got restricted __be64 const *

The updated get_unaligned_t() uses __unqual_scalar_typeof() to get an
unqualified type. This works correctly for the compilers, but fails for
sparse when the data type is __be64 (or any other __beNN variant).

On sparse runs (C=[12]) __beNN types are annotated with
__attribute__((bitwise)).

That annotation allows sparse to detect incompatible operations on __beNN
variables, but it also prevents sparse from evaluating the _Generic() in
__unqual_scalar_typeof() and map __beNN to a unqualified scalar type, so it
ends up with the default, i.e. the original qualified type of a 'const
__beNN' pointer. That then ends up as the first pointer argument to
builtin_memcpy(), which obviously causes the above sparse warnings.

The sparse git tree supports typeof_unqual() now, which allows to use it
instead of the _Generic() based __unqual_scalar_typeof(). With that sparse
correctly evaluates the unqualified type and keeps the __beNN logic intact.

The downside is that this requires a top of tree sparse build and an old
sparse version will emit a metric ton of incomprehensible error messages
before it dies with a segfault.

Therefore implement a sanity check which validates that the checker is
available and capable of handling typeof_unqual(). Emit a warning if not so
the user can take informed action.

[ tglx: Move the evaluation of USE_TYPEOF_UNQUAL to compiler_types.h so it is
  	set before use and implement the sanity checker ]

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Acked-by: Ian Rogers <irogers@google.com>
Link: https://patch.msgid.link/87ecnp2zh3.ffs@tglx
Closes: https://lore.kernel.org/oe-kbuild-all/202601150001.sKSN644a-lkp@intel.com/

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