target/i386: reimplement fyl2x using floatx80 operations

target/i386: reimplement fyl2x using floatx80 operations

The x87 fyl2x emulation is currently based around conversion to
double.  This is inherently unsuitable for a good emulation of any
floatx80 operation.  Reimplement using the soft-float operations,
building on top of the reimplementation of fyl2xp1 and factoring out
code to be shared between the two instructions.

The included test assumes that the result in round-to-nearest mode
should always be one of the two closest floating-point numbers to the
mathematically exact result (including that it should be exact, in the
exact cases which cover more cases than for fyl2xp1).

Signed-off-by: Joseph Myers <joseph@codesourcery.com>
Message-Id: <alpine.DEB.2.21.2006172321530.20587@digraph.polyomino.org.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>