selftests/openat2: fix clang build failures: -static-libasan, LOCAL_HDRS

When building with clang via:

    make LLVM=1 -C tools/testing/selftests

two distinct failures occur:

1) gcc requires -static-libasan in order to ensure that Address
Sanitizer's library is the first one loaded. However, this leads to
build failures on clang, when building via:

       make LLVM=1 -C tools/testing/selftests

However, clang already does the right thing by default: it statically
links the Address Sanitizer if -fsanitize is specified. Therefore, fix
this by simply omitting -static-libasan for clang builds. And leave
behind a comment, because the whole reason for static linking might not
be obvious.

2) clang won't accept invocations of this form, but gcc will:

    $(CC) file1.c header2.h

Fix this by using selftests/lib.mk facilities for tracking local header
file dependencies: add them to LOCAL_HDRS, leaving only the .c files to
be passed to the compiler.

Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>