Merge from branches/dev/axel/cintfastalloc:

* Improve G__FastAllocString; now adds <0.5% (aka noise) instructions for stress.cxx(30); uses almost no additional statics and only <300k heap and reduces the stack size considerably.
* Replace almost all CINT static, fixed sized buffers by G__FastAllocString. This removes many, many possible buffer overflows when reading into fixed sized buffers without checking the buffer length.
* add G__x86_64__ and G__amd64 macros if available


git-svn-id: http://root.cern.ch/svn/root/trunk@29649 27541ba8-7e3a-0410-8455-c3a389f83636