Update the ARMv6 core clang targets to be an arm1176jzf-s. This brings us
in line with gcc in base as this makes llvm generate code for the armv6k
variant of the instruction set.
Simplify the selection of the pvo_head and pvo allocation zone in
moea_enter_locked() and moea64_enter().

Eliminate an unused variable from moea64_enter().