Execute-only for macppc G5

Execute-only for macppc G5

The G5 PowerPC 970 has a Data Address Compare mechanism that can trap
loads and stores to pages with PTE_AC_64, while allowing instruction
fetches.  Use this for execute-only mappings, like we do on powerpc64.

Add a check to pte_spill_v for execute-only mappings.  Without this,
we would forever retry reading an execute-only page.

In altivec_assist, copyin would fail to read the instruction from an
execute-only page.  Add copyinsn to bypass x-only, like sparc64.

with help from abieber@ deraadt@ kettenis@
ok deraadt@