fortify: Add Clang support

fortify: Add Clang support

Enable FORTIFY_SOURCE support for Clang:

Use the new __pass_object_size and __overloadable attributes so that
Clang will have appropriate visibility into argument sizes such that
__builtin_object_size(p, 1) will behave correctly. Additional details
available here:
    https://github.com/llvm/llvm-project/issues/53516
    https://github.com/ClangBuiltLinux/linux/issues/1401

A bug with __builtin_constant_p() of globally defined variables was
fixed in Clang 13 (and backported to 12.0.1), so FORTIFY support must
depend on that version or later. Additional details here:
    https://bugs.llvm.org/show_bug.cgi?id=41459
    commit a52f8a59aef4 ("fortify: Explicitly disable Clang support")

A bug with Clang's -mregparm=3 and -m32 makes some builtins unusable,
so removing -ffreestanding (to gain the needed libcall optimizations
with Clang) cannot be done. Without the libcall optimizations, Clang
cannot provide appropriate FORTIFY coverage, so it must be disabled
for CONFIG_X86_32. Additional details here;
    https://github.com/llvm/llvm-project/issues/53645

Cc: Miguel Ojeda <[email protected]>
Cc: Nick Desaulniers <[email protected]>
Cc: Nathan Chancellor <[email protected]>
Cc: George Burgess IV <[email protected]>
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
Reviewed-by: Nick Desaulniers <[email protected]>
Link: https://lore.kernel.org/r/[email protected]