m88k does not have the m68k long double type actually (long double == double
on this platform), so don't pretend it does, and don't build long double libm
routines.

create new machine/_float.h which is namespace clean. create a new
MI float.h which pulls in and defines the values that are needed from
there, and repair sys/limits.h so that it defines the values

create new machine/_float.h which is namespace clean. create a new
MI float.h which pulls in and defines the values that are needed from
there, and repair sys/limits.h so that it defines the values it needs
as well (depending on POSIX version, XPG version, etc). guenther has
a more exact selection of that coming for limits.h.
this also fixes a few mistakes for the vax.
reviewed by kettenis and guenther.

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