[gdb/build] Fix build with gcc 4.8.5

When building gdb with gcc 4.8.5, we run into problems due to unconditionally
using:
...
     gdb_static_assert (std::is_trivially_copyable<packed>::value);
...
in gdbsupport/packed.h.

Fix this by guarding the usage with HAVE_IS_TRIVIALLY_COPYABLE.

Tested by doing a full gdb build with gcc 4.8.5.