sparc: Re-add abi compliant structure support The original code, removed in the "rewrite" patch, was incorrect for large structures, and required dynamic allocation of a trampoline on every ffi_call. Instead, allocate a 4k entry table of all possible structure returns. The table is 80k, but is read-only and dynamically paged, which ought to be better than allocating the trampoline. This is difficult to test with gcc. One can only use -O0 at present. See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63668.