Add support for the RISC-V architecture

boostcpp.jam

@@ -607,7 +607,7 @@ rule address-model ( )
     return <conditional>@boostcpp.deduce-address-model ;
 }
 
-local deducable-architectures = arm mips1 power sparc x86 combined ;
+local deducable-architectures = arm mips1 power riscv sparc x86 combined ;
 feature.feature deduced-architecture : $(deducable-architectures) : propagated optional composite hidden ;
 for a in $(deducable-architectures)
 {
@@ -618,11 +618,12 @@ rule deduce-architecture ( properties * )
 {
     local result ;
     local filtered = [ toolset-properties $(properties) ] ;
-    local names = arm mips1 power sparc x86 combined ;
+    local names = arm mips1 power riscv sparc x86 combined ;
     local idx = [ configure.find-builds "default architecture" : $(filtered)
         : /boost/architecture//arm
         : /boost/architecture//mips1
         : /boost/architecture//power
+        : /boost/architecture//riscv
         : /boost/architecture//sparc
         : /boost/architecture//x86
         : /boost/architecture//combined ] ;