Profiling: measure time on non-Windows/POSIX using clock_gettime (wip)

PowerPC CPU profiling was only enabled on Windows (using QueryPerformanceCounter). Try to do the same thing using POSIX clock_gettime.
Header files should have a `#pragma once` in them below the license comment.

e.g.

```cpp
// Copyright 2014 Dolphin Emulator Project
// Licensed under GPLv2
// Refer to the license.txt file included.

#pragma once

/* rest of code/definitions */
```