[yaml2obj] - Don't crash when `FileHeader` declares an empty `Flags` key in specific situations.

We currently call the `llvm_unreachable` for the following YAML:

```
--- !ELF
FileHeader:
  Class:   ELFCLASS32
  Data:    ELFDATA2LSB
  Type:    ET_REL
  Machine: EM_NONE
  Flags:   [ ]
```

it happens because the `Flags` key is present, though `EM_NONE` is a
machine type that has no known `EF_*` values and we call `llvm_unreachable` by mistake.

Differential revision: https://reviews.llvm.org/D86138