Float parameter passing in funcall on ppc-aix & ppc-lynx178.

Given the following code:

    float global_float = 0.0;

    void
    set_float (float f)
    {
      global_float = f;
    }

GDB incorrectly calls set_float if the set_float function is marked
as prototyped:

    (gdb) call set_float (5.0)
    (gdb) print global_float
    $1 = 2048

What happens, when the function is marked as prototyped, is that
GDB finds that the argument is a float, casts the value given in
the expression to a float, and then gives that float to ppc-aix/
ppc-lynx178's push_dummy_call gdbarch routine. The latter then blindly
copies it as is in the first floating-point register, instead of
making sure that it has the proper format first.

gdb/ChangeLog:

        * rs6000-aix-tdep.c (rs6000_push_dummy_call): Convert
        floating point registers to register type before storing
        value.
        * rs6000-lynx178-tdep.c (rs6000_lynx178_push_dummy_call):
        Likewise.