coroutine: annotate coroutine_fn for libclang

coroutine: annotate coroutine_fn for libclang

Clang has a generic __annotate__ attribute that can be used by
static analyzers to understand properties of functions and
analyze the control flow.  Furthermore, unlike TSA annotations, the
__annotate__ attribute applies to function pointers as well.

As a first step towards static analysis of coroutine_fn markers,
attach the attribute to the marker when compiling with clang.

Signed-off-by: Alberto Faria <afaria@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20221216110758.559947-2-pbonzini@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>