[llvm-readobj] - Print "Unknown" when a program header is unknown.

Currently, when a program header type is unknown, we dont print anything:

```
ProgramHeader {
  Type:  (0x60000000)
```

With this patch the output will be:

```
ProgramHeader {
  Type: Unknown (0x60000000)
```

It was discussed in D85526 and consistent with what we print for
'--sections' already, e.g.:

```
Section {
  Name: .sec
  Type: Unknown (0x7FFFFFFF)
}
```

Differential revision: https://reviews.llvm.org/D86213