selftests/fchmodat2: fix clang build failure due to -static-libasan

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,
simply omit -static-libasan for clang builds. And leave behind a
comment, because the whole reason for static linking might not be
obvious.

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