Proper IEEE overflow semantics for VMS.

Proper IEEE overflow semantics for VMS.

Way back in 67597c89125e7e14 we misspelled _IEEE_FP as __IEEE_FP,
with a spurious leading underscore.  Which I then copied and
pasted into pp_pack.c in baf3cf9c09c529.  This means that on
Alpha and Itanium systems with the default selection of IEEE
floating point, we've actually (for the last decade!) been
using a workaround intended for VAX or Alpha and Itanium builds
that have explicitly selected VAX-compatible floating point
formats.  Oh well.