Compiler Attributes: Add __counted_by macro

In an effort to annotate all flexible array members with their run-time
size information, the "element_count" attribute is being introduced by
Clang[1] and GCC[2] in future releases. This annotation will provide
the CONFIG_UBSAN_BOUNDS and CONFIG_FORTIFY_SOURCE features the ability
to perform run-time bounds checking on otherwise unknown-size flexible
arrays.

Even though the attribute is under development, we can start the
annotation process in the kernel. This requires defining a macro for
it, even if we have to change the name of the actual attribute later.
Since it is likely that this attribute may change its name to "counted_by"
in the future (to better align with a future total bytes "sized_by"
attribute), name the wrapper macro "__counted_by", which also reads more
clearly (and concisely) in structure definitions.

[1] https://reviews.llvm.org/D148381
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896

Cc: Bill Wendling <[email protected]>
Cc: Qing Zhao <[email protected]>
Cc: Gustavo A. R. Silva <[email protected]>
Cc: Nick Desaulniers <[email protected]>
Cc: Nathan Chancellor <[email protected]>
Cc: Tom Rix <[email protected]>
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
Reviewed-by: Gustavo A. R. Silva <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Acked-by: Miguel Ojeda <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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