Build a universal JNI binary for macos (#106)

Universal libraries embed both arm64 and x64 binaries.

jni/jvm/darwin/build.gradle.kts

@@ -12,12 +12,8 @@ dependencies {
 val copyJni by tasks.creating(Sync::class) {
     onlyIf { org.gradle.internal.os.OperatingSystem.current().isMacOsX }
     dependsOn(":jni:jvm:buildNativeHost")
-    val arch = when (System.getProperty("os.arch")) {
-        "aarch64" -> "aarch64"
-        else -> "x86_64"
-    }
     from(rootDir.resolve("jni/jvm/build/darwin/libsecp256k1-jni.dylib"))
-    into(buildDir.resolve("jniResources/fr/acinq/secp256k1/jni/native/darwin-$arch"))
+    into(buildDir.resolve("jniResources/fr/acinq/secp256k1/jni/native/darwin"))
 }
 
 (tasks["processResources"] as ProcessResources).apply {