New RISC-V port (#281)

* Add RISC-V support

This patch adds support for the RISC-V architecture (https://riscv.org).

This patch has been tested using QEMU user-mode emulation and GCC 7.2.0
in the following configurations:

* -march=rv32imac -mabi=ilp32
* -march=rv32g -mabi=ilp32d
* -march=rv64imac -mabi=lp64
* -march=rv64g -mabi=lp64d

The ABI currently can be found at
https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md .

* Add RISC-V to README

* RISC-V: fix configure.host

src/riscv/ffitarget.h

@@ -0,0 +1,68 @@
+/* -----------------------------------------------------------------*-C-*-
+   ffitarget.h - 2014 Michael Knyszek
+
+   Target configuration macros for RISC-V.
+
+   Permission is hereby granted, free of charge, to any person obtaining
+   a copy of this software and associated documentation files (the
+   ``Software''), to deal in the Software without restriction, including
+   without limitation the rights to use, copy, modify, merge, publish,
+   distribute, sublicense, and/or sell copies of the Software, and to
+   permit persons to whom the Software is furnished to do so, subject to
+   the following conditions:
+
+   The above copyright notice and this permission notice shall be included
+   in all copies or substantial portions of the Software.
+
+   THE SOFTWARE IS PROVIDED ``AS IS'', WITHOUT WARRANTY OF ANY KIND,
+   EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+   MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+   NONINFRINGEMENT.  IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+   HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+   WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+   OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+   DEALINGS IN THE SOFTWARE.
+
+   ----------------------------------------------------------------------- */
+
+#ifndef LIBFFI_TARGET_H
+#define LIBFFI_TARGET_H
+
+#ifndef LIBFFI_H
+#error "Please do not include ffitarget.h directly into your source.  Use ffi.h instead."
+#endif
+
+#ifndef __riscv
+#error "libffi was configured for a RISC-V target but this does not appear to be a RISC-V compiler."
+#endif
+
+#ifndef LIBFFI_ASM
+
+typedef unsigned long ffi_arg;
+typedef   signed long ffi_sarg;
+
+/* FFI_UNUSED_NN and riscv_unused are to maintain ABI compatibility with a
+   distributed Berkeley patch from 2014, and can be removed at SONAME bump */
+typedef enum ffi_abi {
+    FFI_FIRST_ABI = 0,
+    FFI_SYSV,
+    FFI_UNUSED_1,
+    FFI_UNUSED_2,
+    FFI_UNUSED_3,
+    FFI_LAST_ABI,
+
+    FFI_DEFAULT_ABI = FFI_SYSV
+} ffi_abi;
+
+#endif /* LIBFFI_ASM */
+
+/* ---- Definitions for closures ----------------------------------------- */
+
+#define FFI_CLOSURES 1
+#define FFI_TRAMPOLINE_SIZE 24
+#define FFI_NATIVE_RAW_API 0
+#define FFI_EXTRA_CIF_FIELDS unsigned riscv_nfixedargs; unsigned riscv_unused;
+#define FFI_TARGET_SPECIFIC_VARIADIC
+
+#endif
+