C++ PATCH for c++/69657 (abs not inlined)

[Jason Merrill]

The issue in this bug was that due to changes in the libstdc++ headers, 
the built-in abs declaration was getting hidden by a using-declaration, 
so that then when the built-in got an explicit declaration, the original 
declaration wasn't there anymore and so the new declaration didn't get 
marked as built-in.

Fixed by overloading an anticipated built-in rather than clobbering it 
when we see a using-declaration of the same name.

Tested x86_64-pc-linux-gnu, applying to trunk.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: 69657.patch
Type: text/x-patch
Size: 3143 bytes
Desc: not available
URL: <http://gcc.gnu.org/pipermail/gcc-patches/attachments/20160208/d18956e6/attachment.bin>