Define the riscv architecture feature (#424)

src/tools/features/architecture-feature.jam

@@ -30,6 +30,9 @@ feature.feature architecture
         # Advanced RISC Machines
         arm
 
+        # RISC-V
+        riscv
+
         # Combined architectures for platforms/toolsets that support building for
         # multiple architectures at once. "combined" would be the default multi-arch
         # for the toolset.