Implement OS_TaskControl (+ other small improvements)
Some low-hanging fruit from some of my many WIP projects.
- Define RISCOS_KERNEL when building the kernel (for consistency with RiscOS/BuildSys!52 (merged))
- Make sure that (when we enable it) we use the kernel build of SyncLib instead of the app version
- Align HAL workspace to a doubleword boundary, to allow HALs to more easily use LDREXD/STREXD within their workspace
- Implement ROL's OS_TaskControl 0 and make the kernel use it internally