a minor x86 bug reported

[Samuel A. Figueroa]

Perhaps a slightly different kind of "bug" is the kind described in the second
edition of "PA-RISC 1.1 Architecture and Instruction Set Reference Manual" on
pages 6-32, 6-34, and 6-35: when overflow or underflow occurs during floating-
point conversion operations, the unimplemented exception may be signaled in
certain circumstances instead of the overflow or underflow exception, as
required by the IEEE Standard.