[yaml2obj][test] Simplify some e_machine EI_CLASS EI_DATA tests

When both little-endian and big-endian are tested, or both 32-bit and 64-bit are tested, use a template like the following with `-D BITS=32 -D ENCODE=LSB`

```
--- !ELF
FileHeader:
 Class:   ELFCLASS[[BITS]]
 Data:    ELFDATA2[[ENCODE]]
 Type:    ET_DYN
 Machine: EM_X86_64
```

Reviewed By: grimar, jhenderson

Differential Revision: https://reviews.llvm.org/D73828