[Gcov] Unbreak C++ coverage

[Nathan Sidwell]

I've committed this patch, which unbreaks the firefox build problem Markus 
found.  The problem is that the list of functions to emit coverage data is 
determined before the final culling of functions that don't need emitting. 
There's a circular dependency here with the cgraph machinery, and I need to look 
carefully as to how that might be broken.

This does revert some of the new features I was implementing, and the xfailed 
gcov test is a case of this.  I decided to simply remove the tests checking 
coverage object visibility, rather than augment the scan-assembler machinery.

tested on i686-pc-linux-gnu.  Thanks to Markus for verifying this patch does 
indeed unbreak firefox.

nathan
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