[gcov] Erase stale annotated source

[Nathan Sidwell]

I've committed this gcov patch, which does 2 things:

*) if a source file contains no code, erase any annotation file for it.  Such a 
file will be misleading as it will refer to a previous instrumented program.

*) dynamically allocate the source line buffer, rather than use a fixed length 
buffer.

nathan
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