Add some of the missing parts to libkir.h.

Fix bug that caused over-allocation of .text16 and .data16 memory areas
by a factor of 16.

REAL_CODE() don't need it.

completely.

the only one we actually use).  This allows REAL_EXEC fragments to
contain proper references to constraints (e.g. "%w0"), rather than having
to force the use of specific registers.

Note that the "num_constraints" parameter is now completely obsolete, and
that we can probably reduce the syntax to something like
  __asm__ __volatile__ ( REAL_CODE ( "asm statements" )
                         : output constraints
                         : input constraints
                         : clobber );
which would look much more natural, and avoid the need to always specify
a clobber list.


Add userptr_t to libkir.h, to allow it to at least compile.

usual __{text,data}16 trick work with arrays.  gcc seems to accept the
__asm__ ( asmlabel ) only after the [] of the array declaration, not
before.

Use .text16.data section with "aw" attributes, to avoid section type
conflicts when placing both code and data into .text16.

Add __from_{text16,data16}.

interrupt vectors, for example, will be easiest to handle if placed in
.text16.

.data16.  librm will need to supply "char *data16", i.e. the virtual
address of the start of .data16.

Added REAL_FRAGMENT(), because it could conceivably be useful.