future: do not use "cold" attribute for lambda

to workaround GCC-9 bug:

../../include/seastar/core/future.hh: In member function ‘void
seastar::promise<T>::abandoned()’:
../../include/seastar/core/future.hh:1350:38: error: expected identifier
before ‘{’ token
 1350 |         [&] () __attribute__((cold)) {
      |                                      ^
../../include/seastar/core/future.hh:1350:38: error: type-specifier
invalid in lambda

see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89640

Signed-off-by: Kefu Chai <[email protected]>

include/seastar/core/future.hh

@@ -1347,7 +1347,7 @@ void promise<T...>::abandoned() noexcept {
         assert(_state && !_state->available());
         // Encourage the compiler to move this away from the hot paths. __builtin_expect is not enough
         // to do that. Cold lambdas work (at least for GCC8+).
-        [&] () __attribute__((cold)) {
+        [&] () {
             try {
                 // Constructing broken_promise may throw (std::logic_error ctor is not noexcept).
                 set_exception(broken_promise{});