Add a global variable to provide emulator capabilities

[mpharoah]

It would be nice to have a global variable bitfield that we can use to check if the current emulator (or console) supports a particular feature. This is much more robust than checking for a specific emulator version, but only some checks will actually be possible since many features will simply crash inaccurate emulators (especially PJ64) instead of just having different behaviour.

But for those features that we can safely check for, we should provide a way to directly feature test for them instead of relying on gEmulator.

Some potential features we could test for:

• Cache emulation (we already have a check for this)
• Floating point rounding mode support
• emulator extensions once they are standardized (will need a PJ64 1.6 check around this to avoid a crash lol)
• libpl
• DMA timings
• RSP pipeline stall timings
• Certain 64-bit mode behaviors