Add detection of RISC-V from __riscv (#97)

include/boost/predef/architecture.h

@@ -21,6 +21,7 @@ http://www.boost.org/LICENSE_1_0.txt)
 #include <boost/predef/architecture/ppc.h>
 #include <boost/predef/architecture/ptx.h>
 #include <boost/predef/architecture/pyramid.h>
+#include <boost/predef/architecture/riscv.h>
 #include <boost/predef/architecture/rs6k.h>
 #include <boost/predef/architecture/sparc.h>
 #include <boost/predef/architecture/superh.h>

include/boost/predef/architecture/riscv.h

@@ -0,0 +1,41 @@
+/*
+Distributed under the Boost Software License, Version 1.0.
+(See accompanying file LICENSE_1_0.txt or copy at
+http://www.boost.org/LICENSE_1_0.txt)
+*/
+
+#ifndef BOOST_PREDEF_ARCHITECTURE_RISCV_H
+#define BOOST_PREDEF_ARCHITECTURE_RISCV_H
+
+#include <boost/predef/version_number.h>
+#include <boost/predef/make.h>
+
+/*`
+[heading `BOOST_ARCH_RISCV`]
+
+[@http://en.wikipedia.org/wiki/RISC-V] architecture.
+
+[table
+    [[__predef_symbol__] [__predef_version__]]
+
+    [[`__riscv`] [__predef_detection__]]
+    ]
+ */
+
+#define BOOST_ARCH_RISCV BOOST_VERSION_NUMBER_NOT_AVAILABLE
+
+#if defined(__riscv)
+#   undef BOOST_ARCH_RISCV
+#   define BOOST_ARCH_RISCV BOOST_VERSION_NUMBER_AVAILABLE
+#endif
+
+#if BOOST_ARCH_RISCV
+#   define BOOST_ARCH_RISCV_AVAILABLE
+#endif
+
+#define BOOST_ARCH_RISCV_NAME "RISC-V"
+
+#endif
+
+#include <boost/predef/detail/test.h>
+BOOST_PREDEF_DECLARE_TEST(BOOST_ARCH_RISCV,BOOST_ARCH_RISCV_NAME)