Recognize FreeBSD specific BFD names in OUTPUT_FORMAT

Summary:
After rLLD344952 ("Add OUTPUT_FORMAT linker script directive support"),
using BFD names such as `elf64-x86-64-freebsd` the `OUTPUT_FORMAT`
linker script command does not work anymore, resulting in errors like:

```
ld: error: /home/dim/src/clang800-import/stand/efi/loader/arch/amd64/ldscript.amd64:2: unknown output format name: elf64-x86-64-freebsd
>>> OUTPUT_FORMAT("elf64-x86-64-freebsd", "elf64-x86-64-freebsd", "elf64-x86-64-freebsd")
>>>               ^
```

To fix this, recognize a `-freebsd` suffix in BFD names, and also set
`Configuration::OSABI` to `ELFOSABI_FREEBSD` for those cases.

Add and/or update several test cases to check for the correct results of
these new `OUTPUT_FORMAT` arguments.

Reviewers: ruiu, atanasyan, grimar, hokein, emaste, espindola

Reviewed By: ruiu

Subscribers: nemanjai, javed.absar, arichardson, krytarowski, kristof.beyls, kbarton, llvm-commits

Differential Revision: https://reviews.llvm.org/D57283

llvm-svn: 352606