objtool: add UACCESS exceptions for __tsan_volatile_read/write

A lot of the tsan helpers are already excempt from the UACCESS warnings,
but some more functions were added that need the same thing:

kernel/kcsan/core.o: warning: objtool: __tsan_volatile_read16+0x0: call to __tsan_unaligned_read16() with UACCESS enabled
kernel/kcsan/core.o: warning: objtool: __tsan_volatile_write16+0x0: call to __tsan_unaligned_write16() with UACCESS enabled
vmlinux.o: warning: objtool: __tsan_unaligned_volatile_read16+0x4: call to __tsan_unaligned_read16() with UACCESS enabled
vmlinux.o: warning: objtool: __tsan_unaligned_volatile_write16+0x4: call to __tsan_unaligned_write16() with UACCESS enabled

As Marco points out, these functions don't even call each other
explicitly but instead gcc (but not clang) notices the functions
being identical and turns one symbol into a direct branch to the
other.

Link: https://lkml.kernel.org/r/20230215130058.3836177-4-arnd@kernel.org
Fixes: 75d75b7a ("kcsan: Support distinguishing volatile accesses")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Kuan-Ying Lee <Kuan-Ying.Lee@mediatek.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/objtool/check.c

@@ -1186,6 +1186,8 @@ static const char *uaccess_safe_builtin[] = {
 	"__tsan_atomic64_compare_exchange_val",
 	"__tsan_atomic_thread_fence",
 	"__tsan_atomic_signal_fence",
+	"__tsan_unaligned_read16",
+	"__tsan_unaligned_write16",
 	/* KCOV */
 	"write_comp_data",
 	"check_kcov_mode",