Add USE_CPU_INTRINSICS build option?

[illwieckz]

When merging the SSE code from #1142:

• qmath: micro-optimize the BoxOnPlaneSide function DaemonEngine/Daemon#1142

I found myself in the need to test both code, but for that I had to modify the code to change the ifdef.

Using system-dependant ifdefs means we usually don't test the non-intrinsics code.

Even our CI has no configuration to compile the non-intrinsics code, assuming it always run on amd64.

So I suggest a USE_CPU_INTRINSICS CMake option that would enable related definitions, that would be used like in code to disable code specific to some CPU or CPU extension.

The purpose of this option would not be to disable all intrinsics code, for example its purpose would not be to disable intrinsics code in GLM, something we can assume is well tested by others, it would be for the purpose of testing our own code.

In fact all the non-SSE code shared with the sgame/cgame is actually tested when building and running NaCl games, but only in that condition.

I think it is good to have an explicit option to explicitely build and run the generic code.

[illwieckz]

Also, the option would not be to disable automatically enabled compiler optimizations, and would not disable the default compiler flags (we already have USE_CPU_RECOMMENDED_FEATURES=NO for the later).