compiler-clang.h: define __SANITIZE_*__ macros only when undefined

compiler-clang.h: define __SANITIZE_*__ macros only when undefined

Clang 22 recently added support for defining __SANITIZE__ macros similar
to GCC [1], which causes warnings (or errors with CONFIG_WERROR=y or W=e)
with the existing defines that the kernel creates to emulate this behavior
with existing clang versions.

  In file included from <built-in>:3:
  In file included from include/linux/compiler_types.h:171:
  include/linux/compiler-clang.h:37:9: error: '__SANITIZE_THREAD__' macro redefined [-Werror,-Wmacro-redefined]
     37 | #define __SANITIZE_THREAD__
        |         ^
  <built-in>:352:9: note: previous definition is here
    352 | #define __SANITIZE_THREAD__ 1
        |         ^

Refactor compiler-clang.h to only define the sanitizer macros when they
are undefined and adjust the rest of the code to use these macros for
checking if the sanitizers are enabled, clearing up the warnings and
allowing the kernel to easily drop these defines when the minimum
supported version of LLVM for building the kernel becomes 22.0.0 or newer.

Link: https://lkml.kernel.org/r/20250902-clang-update-sanitize-defines-v1-1-cf3702ca3d92@kernel.org
Link: https://github.com/llvm/llvm-project/commit/568c23bbd3303518c5056d7f03444dae4fdc8a9c
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Bill Wendling <morbo@google.com>
Cc: Dmitriy Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>