Please add support for riscv64

[marillat@…]

I noticed issue ​https://trac.mplayerhq.hu/ticket/2344

With this patch mplayer build fine. Build machine is an orange Pi RV2

--- a/configure
+++ b/configure
@@ -288,6 +288,12 @@ arm() {
     *) return 1;;
   esac
 }
+riscv64() {
+  case "$host_arch" in
+    riscv64*) return 0;;
+    *) return 1;;
+  esac
+}
 
 # Use this before starting a check
 echocheck() {
@@ -1870,6 +1876,7 @@ if test -z "$_target" ; then
       nios2) host_arch=nios2 ;;
       vax) host_arch=vax ;;
       xtensa*) host_arch=xtensa ;;
+      riscv64*) host_arch=riscv64 ;;
       *) host_arch=UNKNOWN ;;
   esac
   }
@@ -2591,6 +2598,14 @@ case "$host_arch" in
     if test "$_runtime_cpudetection" = no ; then
       cflag_check -march=native -mcpu=native && _march="-march=native -mcpu=native"
     fi
+    ;;
+
+  riscv64)
+    arch='riscv64'
+    iproc='riscv64'
+    if test "$_runtime_cpudetection" = no ; then
+      cflag_check -march=native -mcpu=native && _march="-march=native -mcpu=native"
+    fi
     ;;
 
   arm*)

[reimar]

What about assembler optimizations? This seems not enough to enable them in e.g. FFmpeg if they exist.