target/ppc: Split up float_invalid_op_excp

The always_inline trick only works if the function is always
called from the outer-most helper.  But it isn't, so pass in
the outer-most return address.  There's no need for a switch
statement whose argument is always a constant.  Unravel the
switch and goto via more helpers.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>