[PowerPC] Don't overwrite vm.pmap sysctl node

[luporl]

Summary

After r367417, both mmu_oea64 and mmu_radix were defining the vm.pmap
sysctl node, resulting in the later definition hiding the properties of
the previous one. Avoid this issue by defining vm.pmap in a common
source file and declaring it where needed.

This change also standardizes the tunable name used to enable superpages.

Test Plan

Checked that all child nodes and properties of vm.pmap can be seen and that the value of superpages_enabled is correct, with the following boot arguments:

• <none>
• vm.pmap.superpages_enabled=1
• radix_mmu=1
• radix_mmu=1 vm.pmap.superpages_enabled=0

[jhibbits]

sys/powerpc/powerpc/pmap_dispatch.c

86

Since superpages still have issues in Radix, it's probably better to just disable it by default for now.

[bdragon]

Looks good to me, and works nicely.

And yeah, I think we should turn off radix superpages for the moment, there are still some weird interactions between it and stuff like the python3 jit.

[luporl]

Leave superpages disabled by default