On amd64, arm64 and i386, we have the possibility to switch between TLS areas in userspace. The nice thing about this is that it makes it easier to do light-weight threading, if we ever feel like doing that. On armv6, let's go into the same direction by making it possible to safely use the TPIDRURW register, which is intended for this purpose.
Clean up the ARMv6 code to remove md_tp entirely. Simply add a dedicated field to the PCB to hold the value of TPIDRURW across context switches, like we do for any other register. As userspace currently uses the read-only TPIDRURO register, simply ensure that we keep both values in sync where possible. The system calls for modifying the read-only register will simply write the intended value into both registers, so that it lazily ends up in the PCB during the next context switch.