Fun fact about all compilers available for PowerPCs running Mac OS X:
they ALL choke on variables defined after the prologue in strict C99
mode.

Tested on:

- CodeWarrior 7.1
- CodeWarrior 8.3
- Apple GCC 3.3
- Apple GCC 4.0

Instead of having each subsystem check whether or not to tick itself,
have step_emulator do it.  Minor performance impact on x86_64 (0.04x),
severe performance impact on ppc (>0.1x).

This has a moderate performance impact on x86_64 and a severe
performance impact on PowerPC.

http://blog.foxkit.us/2015/09/the-joys-of-unix-programming.html

Fix SDL2 build on Mac OS X.

In at least the Windows 7 and Windows 10 SDKs, stat is already a valid function and _stat is used as a private hidden method internally called by stat.  By definiting stat as _stat, it caused an infinite recursion.

(There are five others reported by MSVC 2015 that make no sense.)

Ref #29

Fixes #29 - the problem caused by it, but not the underlying design
issue, maybe.

SDL2, on Mac OS X only, does some really bizarre fucko shit with bools.
Like, there's no other way to describe it.

This fixes it with a stupid yet hilarious hack involving checking if we
have mach/clock.h in a module we don't use it in just to check if OS X
is present, since we don't currently HAVE a platform definition for OS
X, it's "just POSIX".

It Just Werks®, right?

execute is a very heavy call (lots of stack regs being pushed and
popped), so don't call it when we don't have to

This was causing a mess of warnings on MSVC now that /W3 from CMake is actually working, so I wanted to fix it.

Modulus is extremely slow, so use a counter instead when a write-back is
needed. When the data is written, reset the counter.

This offers a speedup at lower emulation levels and may shut up some
static analysis tools

This separates MMIO from regular memory reads. It also reduces clutter
in hardware-specific files.

On my TODO list is perhaps use a special #define to alter visibilitiy of
I/O functions, so they can go back into their own hardware-specific
files. Not yet, though...