stage2: lower unnamed constants in Elf and MachO

* link: add a virtual function `lowerUnnamedConsts`, similar to
  `updateFunc` or `updateDecl` which needs to be implemented by the
  linker backend in order to be used with the `CodeGen` code
* elf: implement `lowerUnnamedConsts` specialization where we
  lower unnamed constants to `.rodata` section. We keep track of the
  atoms encompassing the lowered unnamed consts in a global table
  indexed by parent `Decl`. When the `Decl` is updated or destroyed,
  we clear the unnamed consts referenced within the `Decl`.
* macho: implement `lowerUnnamedConsts` specialization where we
  lower unnamed constants to `__TEXT,__const` section. We keep track of the
  atoms encompassing the lowered unnamed consts in a global table
  indexed by parent `Decl`. When the `Decl` is updated or destroyed,
  we clear the unnamed consts referenced within the `Decl`.
* x64: change `MCValue.linker_sym_index` into two `MCValue`s: `.got_load` and
  `.direct_load`. The former signifies to the emitter that it should
  emit a GOT load relocation, while the latter that it should emit
  a direct load (`SIGNED`) relocation.
* x64: lower `struct` instantiations

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