rust: add --rust-target option for bindgen

rust: add --rust-target option for bindgen

Without it, recent bindgen will give an error

   error: extern block cannot be declared unsafe

if rustc is not new enough to support the "unsafe extern" construct.

Cc: qemu-rust@nongnu.org
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20250206111514.2134895-1-pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit 131c58469f6fb68c89b38fee6aba8bbb20c7f4bf)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>