Build a universal JNI binary for macos (#106)

Universal libraries embed both arm64 and x64 binaries.

jni/jvm/build.sh

@@ -17,6 +17,7 @@ if [ "$TARGET" == "linux" ]; then
   CC_OPTS="-fPIC"
 elif [ "$TARGET" == "darwin" ]; then
   OUTFILE=libsecp256k1-jni.dylib
+  CC_OPTS="-arch arm64 -arch x86_64"
 elif [ "$TARGET" == "mingw" ]; then
   OUTFILE=secp256k1-jni.dll
   CC=x86_64-w64-mingw32-gcc

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 {

jni/jvm/src/main/kotlin/fr/acinq/secp256k1/jni/OSInfo.kt

@@ -19,7 +19,8 @@ internal object OSInfo {
    private const val PPC = "ppc"
    private const val PPC64 = "ppc64"
 
-   @JvmStatic val nativeSuffix: String get() = "$os-$arch"
+    // on macos we build a universal library that contains arm64 and x64 binaries
+   @JvmStatic val nativeSuffix: String get() = if (os == "darwin") os else "$os-$arch"
 
    @JvmStatic val os: String get() = translateOSName(System.getProperty("os.name"))