File ffcall/avcall/tests.output/mips-sgi-irix6.4-cc-64.README from the latest check-in

On mips-sgi-irix6.4 with "cc -64", avcall does not handle correctly
structure return values which consist of 1 or 2 floats or doubles.
(The return values are found in $f0 and $f2, but avcall expects them
in $2 and $3.)