Implement execute-only mappings by using the Virtual Page Class Key

Implement execute-only mappings by using the Virtual Page Class Key
Protection mechanism provided by modern POWER CPUs.  This is implemented
in a way data allows us to use the Data Address Compare mechanism that
was available on older versions of the architecture if we ever add support
for these older CPUs (e.g. the PowerPC 970 aka G5).

Special thanks to gkoehler@ for spotting the bug in my initial
implementation that made this not work at all.

ok deraadt@, gkoehler@