Build sdk variants of libc++ for riscv64

There is no prebuilt libc++ for riscv64 provided by the NDK, build
it using the platform sources instead.

Test: lunch aosp_riscv64-userdebug && m -k
Change-Id: I990709e4b2a64b43ba2713019533e771a51dc762
diff --git a/Android.bp b/Android.bp
index 13951ff..83b2965 100644
--- a/Android.bp
+++ b/Android.bp
@@ -164,6 +164,10 @@
         "//apex_available:anyapex",
     ],
     min_sdk_version: "apex_inherit",
+    // sdk_version normally isn't needed as only the platform variant
+    // is used, except on riscv64 which doesn't have an NDK libc++ and
+    // reuses this module for sdk variants.
+    sdk_version: "current",
 }
 
 cc_fuzz {