should mark public symbols with [[gnu::visibility("default")]]

[tchaikov]

currently, we don't differentiate the public symbols from the private ones using [[gnu::visibility("default")]]. but would be desirable if we can have a smaller global / public symbol list exposed from seastar libraries for better performance and for smaller disk foot print, and this can potentially allow the compiler emit more efficient code at the link time.

while symbol visibility is theoretically important for shared libraries (DSO), in practice, it's less critical for seastar applications. many of these applications link against the static build of seastar libraries, and only optionally link to the shared libraries in debug builds. therefore, addressing this issue is not a high priority from a practical standpoint.