x86, kcsan: Add __no_kcsan to noinstr

commit 5ddbc4082e1072eeeae52ff561a88620a05be08f upstream.
Backport summary: for 5.4 kernel fsgsbase support.

The 'noinstr' function attribute means no-instrumentation, this should
very much include *SAN. Because lots of that is broken at present,
only include KCSAN for now, as that is limited to clang11, which has
sane function attribute behaviour.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
(cherry picked from commit 5ddbc4082e1072eeeae52ff561a88620a05be08f)
Signed-off-by: Ethan Zhao <Haifeng.Zhao@intel.com>

include/linux/compiler_types.h

@@ -180,6 +180,10 @@ struct ftrace_likely_data {
  */
 #define noinline_for_stack noinline
 
+/* Section for code which can't be instrumented at all */
+#define noinstr								\
+	noinline notrace __attribute((__section__(".noinstr.text")))
+
 #endif /* __KERNEL__ */
 
 #endif /* __ASSEMBLY__ */