Make the direct-map API always available, but fail if KASAN or rump.

(Only for architectures that support it at all; on others,
__HAVE_MM_MD_DIRECT_MAPPED_PHYS/IO are still undefined and the
functions unimplemented.)

This gives modules like zfs an opportunity to use it.

While here, fix the one caller of mm_md_direct_mapped_phys that
ignored the return value (and make sure to call pmap_kremove/update
before uvm_km_free).

branches: 1.8.28;
Rototill a bit, and attempt to disguise it as non-gratuitous.

Add arch/generic and move non-x86 files from rumpkern/ there. Also,
move files from arch/i386 to arch/x86, and make both i386 and x86_64
use those.

This clarifies the situation with what is MD vs. MI code.

renames:
rumpcpu_generic,kobj_stubs,pmap_stubs => arch/generic/rump_generic_$x
arch/i386/* => arch/x86/rump_x86_$x

(for those who forget, x86 requires MD code because rump kernels
use the same ABI as kernel modules)

branches: 1.7.8; 1.7.18; 1.7.22;
Reinstate the blanket pmap.h for archs which do not conform to the
kernel ABI (i.e. not i386 or amd64). Due to the "half function,
half macro, all noodles" nature of pmap.h, it's too entangling and
too brittle to keep up with an ifdeffy MI implementation.

The x86 kernel ABI depends on __cpu_simple_lock stuff being present.
Since they are practically never used (only when prehistoric code
uses simple_lock()), their efficiency doesn't matter that much and
we can simply adapt the versions from x86 lock.h.

add __HAVE_CPU_COUNTER stubs where possible (i.e. where the arch
doesn't think inlines are the second compiling)

branches: 1.4.2; 1.4.4; 1.4.8; 1.4.10; 1.4.12;
add missing "arch/" as noted by a few people

Define MODULAR for rump core components. This enables module
loading via the kernel module framework (instead of dlopen()).
For now it only works on amd64 and i386, but for the rest it should
just be a matter of including the relevant kobj_machdep.c modules
from the kernel sources.

Rename i386 and amd64 {cpu,spl}.c to rump{cpu,spl}.c to avoid
name collisions with other source files in the kernel tree.

Switch amd64 rumps to the kernel ABI.