target/i386: fix fscale handling of infinite exponents

target/i386: fix fscale handling of infinite exponents

The fscale implementation passes infinite exponents through to generic
code that rounds the exponent to a 32-bit integer before using
floatx80_scalbn.  In round-to-nearest mode, and ignoring exceptions,
this works in many cases.  But it fails to handle the special cases of
scaling 0 by a +Inf exponent or an infinity by a -Inf exponent, which
should produce a NaN, and because it produces an inexact result for
finite nonzero numbers being scaled, the result is sometimes incorrect
in other rounding modes.  Add appropriate handling of infinite
exponents to produce a NaN or an appropriately signed exact zero or
infinity as a result.

Signed-off-by: Joseph Myers <joseph@codesourcery.com>
Message-Id: <alpine.DEB.2.21.2005070045010.18350@digraph.polyomino.org.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>