PR go/61877
refect: fix direct call of variadic method value

As reported in bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61877
gcc mainline has regressed in this.  This CL adds the tests proposed
for the main Go repository:

        https://codereview.appspot.com/151280043/
        https://codereview.appspot.com/152060043/

restores the code from the amd64/386 path that makes this work and
was lost when the Go 1.3 stdlib was merged and changes the FFI path
to call into the same helper code as the amd64/386 path.

I've only tested this on amd64 but I did test a version that was
patched to unconditionally take the FFI path.

From-SVN: r215859