1.0.38.9: Integer callback result fixes.

1.0.38.9: Integer callback result fixes.

  * Callback results should be typechecked based on their declared
type, but stored as if they were the full width of a machine register.

  * Fixed sb-alien::alien-callback-lisp-wrapper-lambda to make this
happen properly.

  * Updated corresponding tests (formerly callback.impure.lisp /
sign-extension and underflow-detection) to cover 16-bit cases, which
would have broken on all targets, not merely 64-bit targets.

  * As a minor side note, assistance in testing the changes in 1.0.38.8
was provided by one Andreas Selfjord Eriksen, but I forgot to note this
fact in the commit message.  Mea Culpa.