Fix OS_Byte 166 for high ZeroPage, make read-only
Despite never being properly documented in the RISC OS PRMs, there's evidence that Acorn were recommending that third-parties used OS_Byte 166 for locating the OS_Byte variables well into the mid-1990's:
Currently OS builds which use zero page relocation will only return the low 16 bits of the address from OS_Byte 166, most likely breaking any third-party software which is trying to use it. Attempt to restore compatibility by having OS_Byte 166 add the missing high address bits into the "next location" value returned in R2, and fix OS_Byte 167 in a similar manner (adding into R1).
Additionally, make the values read-only, because the kernel is littered with code that uses hard-coded OsbyteVars addresses and so is unlikely to do anything sensible if someone was to modify the OsbyteVars address that's stored in workspace.