[ELF] Rename OutputSection::sectionCommands to commands. NFC

This partially reverts r315409: the description applies to LinkerScript, but not
to OutputSection.

The name "sectionCommands" is used in both LinkerScript::sectionCommands and
OutputSection::sectionCommands, which may lead to confusion.
"commands" in OutputSection has no ambiguity because there are no other types
of commands.