[llvm-objcopy][ELF] Add --set-section-type

The request is mentioned on D129053. I feel that having this functionality is
mildly useful (not strong).

* Rename .ctors to .init_array and change sh_type to SHT_INIT_ARRAY (GNU objcopy
  detects the special name but we don't).
* Craft tests for a new SHT_LLVM_* extension

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D129337