MFC r256707, r256708, r257291, r258358

  Switch to use WBWA mappings for page tables on armv6, this is needed for SMP.
  Fix PTE_SYNC() for PIPT L2 caches, using the virtual address wasn't so useful.
  Use PTE_SYNC() for >= armv6
  Spell cpu_l2cache_wb_range correctly.

  Fix condition that determines PMAP_NEEDS_PTE_SYNC value for ARM

  Use values of the correct defines to determine statement's result.
  ARM_ARCH_ symbols are always defined, hence only values are relevant.

  Avoid clearing EXEC permission bit when setting the page RW on ARMv6/v7

  When emulating modified bit the executable attribute was cleared by
  mistake when calling pmap_set_prot().