MFC r315697:

libkvm: fix warning issues post-r291406

• Fix -Wunused warnings with *_native detection handlers by marking kd __unused, except with arm/mips, where a slightly more complicated scheme is required to handle the native case vs the non-native case.
• Fix -Wmissing-variable-declarations warnings by marking struct kvm_arch objects static.

Tested with: WIP test code (D10024) // kgdb7121 (i386 crash/kernel on amd64)