sparc: Rewrite everything

It's impossible to call between v8 and v9 ABIs, because of the stack bias
in the v9 ABI.  So let's not pretend it's just not implemented yet.  Split
the v9 code out to a separate file.

The register windows prevent ffi_call from setting up the entire stack
frame the assembly, but we needn't make an indirect call back to prep_args.
sparc: Preprocess float point struct return

We can eliminate recursion and speed structure return
by flattening a nested structure tree into a bitmask.
sparc: Handle more cases of structure return directly
sparc: Add support for complex types