compiler_types.h: add "auto" as a macro for "__auto_type"

"auto" was defined as a keyword back in the K&R days, but as a storage
type specifier.  No one ever used it, since it was and is the default
storage type for local variables.

C++11 recycled the keyword to allow a type to be declared based on the
type of an initializer.  This was finally adopted into standard C in
C23.

gcc and clang provide the "__auto_type" alias keyword as an extension
for pre-C23, however, there is no reason to pollute the bulk of the
source base with this temporary keyword; instead define "auto" as a
macro unless the compiler is running in C23+ mode.

This macro is added in <linux/compiler_types.h> because that header is
included in some of the tools headers, wheres <linux/compiler.h> is
not as it has a bunch of very kernel-specific things in it.

[ Cc: stable to reduce potential backporting burden. ]

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Cc: <stable@kernel.org>