Add detection of the RISC-V architecture

checks/architecture/Jamroot.jam

@@ -19,5 +19,6 @@ obj arm      : arm.cpp ;
 obj combined : combined.cpp ;
 obj mips1    : mips1.cpp ;
 obj power    : power.cpp ;
+obj riscv    : riscv.cpp ;
 obj sparc    : sparc.cpp ;
 obj x86      : x86.cpp ;

checks/architecture/riscv.cpp

@@ -0,0 +1,9 @@
+// riscv.cpp
+//
+// 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)
+
+#if !defined(__riscv)
+#error "Not RISC-V"
+#endif