Linux: support cross-compile native library for ARM64 on x86_64 Linux (issue #899)

flatlaf-natives/flatlaf-natives-linux/README.md

@@ -5,12 +5,15 @@ This sub-project contains the source code for the FlatLaf Linux native library.
 
 The native library can be built only on Linux and requires a C++ compiler.
 
+The native library is available for following CPU architectures: `x86_64` (or
+`amd64`) and `arm64` (or `aarch64`).
+
 To be able to build FlatLaf on any platform, and without C++ compiler, the
-pre-built native library is checked into Git at
+pre-built native libraries are checked into Git at
 [flatlaf-core/src/main/resources/com/formdev/flatlaf/natives/](https://github.com/JFormDesigner/FlatLaf/tree/main/flatlaf-core/src/main/resources/com/formdev/flatlaf/natives).
 
-The native library was built on a GitHub server with the help of GitHub Actions.
-See:
+The native libraries were built on a GitHub server with the help of GitHub
+Actions. See:
 [Native Libraries](https://github.com/JFormDesigner/FlatLaf/actions/workflows/natives.yml)
 workflow. Then the produced Artifacts ZIP was downloaded and the native library
 checked into Git.
@@ -18,19 +21,27 @@ checked into Git.
 
 ## Development
 
-To build the library on Linux, some packages needs to be installed.
+To build the library on Linux, some packages needs to be installed:
+
+- `build-essential` - GCC and development tools
+- `libxt-dev` - X11 toolkit development headers
+- `g++-aarch64-linux-gnu` - GNU C++ compiler for the arm64 architecture (only on
+  x86_64 Linux for cross-compiling for arm64 architecture)
 
 
 ### Ubuntu
 
-`build-essential` contains GCC and development tools. `libxt-dev` contains the
-X11 toolkit development headers.
-
 ~~~
 sudo apt update
 sudo apt install build-essential libxt-dev
 ~~~
 
+Only on x86_64 Linux for cross-compiling for arm64 architecture:
+
+~~~
+sudo apt install g++-aarch64-linux-gnu
+~~~
+
 
 ### CentOS