write/coff: support for weak symbols seems wrong

[philipc]

The PE file format specifies that weak symbols should have an auxiliary record: https://docs.microsoft.com/en-us/windows/win32/debug/pe-format#auxiliary-format-3-weak-externals

Additionally, we don't emit undefined weak symbols, and I'm not sure what these should look like. This is similar to a bug for ELF (#247).