- add missing functions SignalCodeMovingGC() and MemoryBarrier() 
- avoid pointer conversion/comparison warnings 
- don't attempt to hide symbols with -fvisibility, MinGW doesn't support it 

BUG=http://code.google.com/p/v8/issues/detail?id=949

Patch by Bert Belder <bertbelder@gmail.com>

Review URL: http://codereview.chromium.org/5471001


git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5913 ce2b1a6d-e550-0410-aec6-3dcde31c8c00