add support for riscv64 (#649)

Author: futurejones

cmake/modules/SwiftSupport.cmake

@@ -37,6 +37,8 @@ function(get_swift_host_arch result_var_name)
     set("${result_var_name}" "i686" PARENT_SCOPE)
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "i686")
     set("${result_var_name}" "i686" PARENT_SCOPE)
+  elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "riscv64")
+    set("${result_var_name}" "riscv64" PARENT_SCOPE)
   else()
     message(FATAL_ERROR "Unrecognized architecture on host system: ${CMAKE_SYSTEM_PROCESSOR}")
   endif()