RISC-V go closures

This implements go closures for RISC-V.  It has been tested on
riscv64-suse-linux and against the libgo testsuite.

src/riscv/ffitarget.h

@@ -59,6 +59,7 @@ typedef enum ffi_abi {
 /* ---- Definitions for closures ----------------------------------------- */
 
 #define FFI_CLOSURES 1
+#define FFI_GO_CLOSURES 1
 #define FFI_TRAMPOLINE_SIZE 24
 #define FFI_NATIVE_RAW_API 0
 #define FFI_EXTRA_CIF_FIELDS unsigned riscv_nfixedargs; unsigned riscv_unused;