compiler-clang: add build check for clang 10.0.1

Patch series "set clang minimum version to 10.0.1", v3.

Adds a compile time #error to compiler-clang.h setting the effective
minimum supported version to clang 10.0.1.  A separate patch has already
been picked up into the Documentation/ tree also confirming the version.

Next are a series of reverts. One for 32b arm is a partial revert.

Then Marco suggested fixes to KASAN docs.

Finally, improve the warning for GCC too as per Kees.

This patch (of 7):

During Plumbers 2020, we voted to just support the latest release of Clang
for now.  Add a compile time check for this.

We plan to remove workarounds for older versions now, which will break in
subtle and not so subtle ways.

Suggested-by: Sedat Dilek <[email protected]>
Suggested-by: Nathan Chancellor <[email protected]>
Suggested-by: Kees Cook <[email protected]>
Signed-off-by: Nick Desaulniers <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Tested-by: Sedat Dilek <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Reviewed-by: Miguel Ojeda <[email protected]>
Reviewed-by: Sedat Dilek <[email protected]>
Acked-by: Marco Elver <[email protected]>
Acked-by: Nathan Chancellor <[email protected]>
Acked-by: Sedat Dilek <[email protected]>
Cc: Andrey Konovalov <[email protected]>
Cc: Fangrui Song <[email protected]>
Cc: Masahiro Yamada <[email protected]>
Cc: Daniel Borkmann <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Vincenzo Frascino <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Link: https://github.com/ClangBuiltLinux/linux/issues/9
Link: https://github.com/ClangBuiltLinux/linux/issues/941
Signed-off-by: Linus Torvalds <[email protected]>

include/linux/compiler-clang.h

@@ -3,6 +3,14 @@
 #error "Please don't include <linux/compiler-clang.h> directly, include <linux/compiler.h> instead."
 #endif
 
+#define CLANG_VERSION (__clang_major__ * 10000	\
+		     + __clang_minor__ * 100	\
+		     + __clang_patchlevel__)
+
+#if CLANG_VERSION < 100001
+# error Sorry, your version of Clang is too old - please use 10.0.1 or newer.
+#endif
+
 /* Compiler specific definitions for Clang compiler */
 
 /* same as gcc, this was present in clang-2.6 so we can assume it works