macosx: Change the optimizations paramaters.

macosx: Change the optimizations paramaters.

From the beginning we were using -march=pentium-m -mtune=prescott
However this was based on the Intel development machine of Apple. All actually shipped Intel machines by Apple are at least Core machines. As such, we might as well use -march=prescott -mtune=generic now. For 10.6 I currently think we should use -march=prescott -mtune=core2, since that will allow us to use much more advanced optimizations, and only few machines were shipped as Core, most were Core2 machines.