Linker plugins should be aware of --defsym during symbol resolution

[Cary Coutant]

> Fair enough. It was a contrived example, not based on anything I have seen
> so far. If that violates ODR then I agree all bets are off.

BTW, Ian wrote this comment in script.cc:

// The GNU linker lets symbol assignments in the linker script
// silently override defined symbols in object files.  We are
// compatible.  FIXME: Should we issue a warning?

I think this is a misfeature, and the comment suggests that Ian thought so too.

-cary