[PowerPC64LE] Implement endian-independent dword atomic PTE lock.

It's much easier to implement this in an endian-independent way when we
don't also have to worry about masking half of the dword off.

Given that this code ran on a machine that ran a poudriere bulk with no
kernel oddities, I am relatively certain it is correctly implemented. ;)

This should be a minor performance boost on BE as well.

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