Fix for AP1 emulation, some code tidying, and reserve an OS_PlatformFeatures 0 bit
- Fix an infinite abort loop when trying to use the LDA instruction with emulated AP1 memory
- Reserve OS_PlatformFeatures 0 bit 22 for Pyromaniac: https://www.riscosopen.org/forum/forums/3/topics/16609
- Add some safety checks to s.CPUFeatures to try and avoid CallASWI reporting the wrong flag values (c.f. RiscOS/Sources/Utilities/Patches/CallASWI!1 (merged))
- Fix a couple of comment typos