Better debugging support in Mach

[Samuel Thibault]

Hello,

At last I got it working: in the latest git master, the bootstrap code
can cope with VM_MIN_KERNEL_ADDRESS != 0 by using segmentation before
switching to C code. This means we can make the kernel live at high
virtual addresses, which means that the debugger trace & such can
properly know how to separate user and kernel addresses.

It works for me on kvm, but I'd prefer to see more tests before enbling
it by default. The attached patch is all that is needed to enable it.

Samuel

patch
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