Previously, the  kernel stack was allocated inside the .kernel_bss
section, along other uninitialized data. While this works, it assumes
that the section will always be laid out with the kernel stack at the
top. If that is not the case, the allocated guard page before
.kernel_bss would not trigger a #PF since out-of-bounds (overflow)
access would quietly overwrite other data in the section, thus
potentially corrupting global kernel data.

At the same time, there was no separate section (and guard page) for the
future #DF exception stack (IST1 in our case). Thus we allocate a
section, and a guard page, while we are already modifying the linker script.

We currently don't really support user mode anymore, since it doesn't
integrate well within the architecture. We therefore need no special
handling of STL and kstd anymore.