[yaml2obj] Add -D k=v to preprocess the input YAML

Examples:

```
yaml2obj -D MACHINE=EM_386 a.yaml -o a.o
yaml2obj -D MACHINE=0x1234 a.yaml -o a.o
```

where a.yaml contains:

```
--- !ELF
FileHeader:
  Class:   ELFCLASS64
  Data:    ELFDATA2MSB
  Type:    ET_REL
  Machine: [[MACHINE]]
```

Reviewed By: grimar, jhenderson

Differential Revision: https://reviews.llvm.org/D73821