Fix panics when target arch is riscv64

Fix panics in api_level.go and apex.go when using riscv64 as the target
arch.

Bug: 250918230
Test: lunch aosp_riscv64-userdebug && m ALLOW_MISSING_DEPENDENCIES=true nothing
Change-Id: I85c7685f3d14fa2dc7ffbcdea7f490feca304ef7

cc/api_level.go

@@ -28,6 +28,8 @@ func minApiForArch(ctx android.BaseModuleContext,
 		return ctx.Config().MinSupportedSdkVersion()
 	case android.Arm64, android.X86_64:
 		return android.FirstLp64Version
+	case android.Riscv64:
+		return android.FutureApiLevel
 	default:
 		panic(fmt.Errorf("Unknown arch %q", arch))
 	}