[MC][ELF] Fix "enumeral and non-enumeral type in conditional expression" warning (NFC)

GCC warning:
```
/llvm-project/llvm/lib/MC/ELFObjectWriter.cpp: In member function ‘void {anonymous}::ELFWriter::writeHeader(const llvm::MCAssembler&)’:
/llvm-project/llvm/lib/MC/ELFObjectWriter.cpp:421:20: warning: enumeral and non-enumeral type in conditional expression [-Wextra]
  420 |   W.OS << char(OSABI == ELF::ELFOSABI_NONE && OWriter.seenGnuAbi()
      |                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  421 |                    ? ELF::ELFOSABI_GNU
      |                    ^~~~~~~~~~~~~~~~~~~
  422 |                    : OSABI);
      |                    ~~~~~~~
```