stand: kick out quad.h

Use quad.h from libc instead for the time being. This reduces the number of
nearly-identical-quad.h we have in tree to two with only minor changes.

Prototypes for some *sh*di3 have been added to match the copy in libkern.
The differences between the two are likely few enough that they can perhaps
be merged with little additional effort to bring us down to 1.

MFC after: 3 days