The trick is to make all destructors non-virtual. This is safe because
we never use the delete operator, which is the only case where virtual
destructors are relevant. This is a better solution than implementing
our own delete operator, because we want libraries loaded at runtime to
use the libstdc++ version, not ours.
We now support immortal objects, which the GC will scan for references
but not consider for collection. On x86_64, we allocate JIT code memory
via mmap, which lets us map memory into the bottom 2GB of the address
space, ensuring that 32-bit relative jumps and calls work.