Disabled preferred stack boundary on x86_64in GSdx.
Fix some _M_AMD64 checks in GSdx.

In the future these will be _M_X86_64, but for now this won't be the case.
Fix function definitions in GSDrawScanlineCodeGenerator.x64

These have been changed since this was last compiled.
Also removes a #error and drops it to a #warning for now while development is in process.
Update GSSetupPrimCodeGenerator for x86_64.

offsetof requires the expression to be constant to get the offset.
Use a combination of offsetof + sizeof to get the correct offset.
Merge pull request #146 from Sonicadvance1/GSdx-x86_64-fixes

Fixes for getting GSdx to compile on x86_64