8285630: Fix a configure error in RISC-V cross build

make/autoconf/build-aux/config.sub

@@ -40,6 +40,13 @@ if echo $* | grep pc-msys >/dev/null ; then
     exit
 fi
 
+# Canonicalize for riscv which autoconf-config.sub doesn't handle
+if echo $* | grep '^riscv\(32\|64\)-linux' >/dev/null ; then
+    result=`echo $@ | sed 's/linux/unknown-linux/'`
+    echo $result
+    exit
+fi
+
 # First, filter out everything that doesn't begin with "aarch64-"
 if ! echo $* | grep '^aarch64-' >/dev/null ; then
     . $DIR/autoconf-config.sub "$@"