Shared mutex pool

[François Dumont]

Hi

     Here is the patch to share a mutex pool between debug mode and 
shared_ptr implementation. It saves 392 bytes on generated .so and will 
make sure that fixing false sharing will impact both usages.

     I preferred to leave implementation in shared_ptr.cc to avoid 
introducing another translation unit.

     * src/c++11/shared_ptr.cc (mask, invalid, get_mutex): Move
     declaration...
     * src/c++11/mutex_pool.h: ... here. New.
     * src/c++11/debug.cc: Use latter.

     Tested under Linux x86_64, normal and debug modes.

Ok to commit ?

François

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