future: avoid calling ~std::exception_ptr
future: avoid calling ~std::exception_ptr

With this the move constructor goes from 12 instructions to just:

    movq    (%rsi), %rax
    movq    %rax, (%rdi)
    movq    $0, (%rsi)
    ret

This assumes that ~exception_ptr does nothing for moved values. That
is the case in libstdc++, so we avoid the call only if __GLIBCXX__ is
defined.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

cloudius-systems/seastar,syuu1228/seastar,avikivity/seastar,dreamsxin/seastar,dreamsxin/seastar,syuu1228/seastar,scylladb/seastar,scylladb/seastar,scylladb/seastar,syuu1228/seastar,avikivity/seastar,cloudius-systems/seastar,avikivity/seastar,dreamsxin/seastar,cloudius-systems/seastar