target/i386: reimplement fpatan using floatx80 operations

The x87 fpatan 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, as
for other such instructions.
Signed-off-by: NJoseph Myers <joseph@codesourcery.com>

Message-Id: <alpine.DEB.2.21.2006230000340.24721@digraph.polyomino.org.uk>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>