backport: ffi.c (ffi_call_go, [...]): New functions.

Backport of RISC-V support for libffi go closures
* src/riscv/ffi.c (ffi_call_go, ffi_prep_go_closure): New
functions.
(ffi_call_int): Renamed from ffi_call.
(ffi_call_asm, ffi_closure_inner): Adjust interface.
* src/riscv/ffitarget.h (FFI_GO_CLOSURES): Define.
* src/riscv/sysv.S (ffi_go_closure_asm): New function.
(ffi_closure_asm, ffi_call_asm): Update for adjusted interfaces.

From-SVN: r263550