We changed powerpc64 to allow write access to write only mappings.

We changed powerpc64 to allow write access to write only mappings.
Although the page table cannot prevent reads on write only pages,
the first access in trap() knows what is is.  This should be passed
to uvm_fault().  Then regress/sys/kern/fork-exit passes.  Copy the
new powerpc64 logic to powerpc.
OK tobhe@ kettenis@ deraadt@