kcsan: Ignore GCC 11+ warnings about TSan runtime support

kcsan: Ignore GCC 11+ warnings about TSan runtime support

GCC 11 has introduced a new warning option, -Wtsan [1], to warn about
unsupported operations in the TSan runtime. But KCSAN != TSan runtime,
so none of the warnings apply.

[1] https://gcc.gnu.org/onlinedocs/gcc-11.1.0/gcc/Warning-Options.html

Ignore the warnings.

Currently the warning only fires in the test for __atomic_thread_fence():

kernel/kcsan/kcsan_test.c: In function ‘test_atomic_builtins’:
kernel/kcsan/kcsan_test.c:1234:17: warning: ‘atomic_thread_fence’ is not supported with ‘-fsanitize=thread’ [-Wtsan]
 1234 |                 __atomic_thread_fence(__ATOMIC_SEQ_CST);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

which exists to ensure the KCSAN runtime keeps supporting the builtin
instrumentation.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>