Support depfiles for ELF linkers

lld and (newer) ld.bfd and ld.gold support a --dependency-file= flag to dump out depfiles. We should hook this up in the Ninja and Makefiles generators.

See this message on the gn-dev list.

Cc: @kyle.edwards @brad.king @marc.chevrier

Edited May 20, 2021 by Ben Boeckel

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