"TLS-lite": add kernel support for a per-thread userspace pointer,

for pointing to the thread-control-block.  Support for mapping this
to the correct hardware register can be added as it's finished;
start with support for amd64, sparc, and sparc64.  Includes syscalls
for getting and setting it (for a portable __errno implementation) as
well as creating a new thread with an initial value for it.

discussed with miod@, kettenis@, deraadt@; committing to get the syscalls
in with the impending libc bump and do further refinements in tree

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