docs: new amd64 compilation instructions

.cargo/config.toml

+[target.aarch64-unknown-linux-gnu]
+ar = "./aarch64-linux-gnu/bin/aarch64-none-linux-gnu-ar"
+linker = "./aarch64-linux-gnu/bin/aarch64-none-linux-gnu-gcc"
+
 [target.aarch64-linux-android]
 ar = "./ndk/arm64/bin/aarch64-linux-android-ar"
 linker = "./ndk/arm64/bin/aarch64-linux-android-clang"

.gitignore

@@ -6,6 +6,7 @@ Cargo.lock
 
 /ndk
 /target
+/aarch64-linux-gnu
 /res/roms.prop
 /frontends/*/target
 /frontends/*/ndk

frontends/libretro/README.md

@@ -6,7 +6,36 @@
 cargo build
 ```
 
-### Android
+### Cross-compiling
+
+#### Arm64
+
+Download the linux toolchain from [https://developer.arm.com/downloads/-/gnu-a](https://developer.arm.com/downloads/-/gnu-a).
+
+Set the env variable `ARM64_TOOLCHAIN` to the path of the toolchain directory.
+
+Create a toolchain synbolic link using the following command in Unix:
+
+```bash
+ln -s $ARM64_TOOLCHAIN aarch64-linux-gnu
+```
+
+... and the following command in Windows (cmd vs powershell):
+
+```bash
+mklink /D aarch64-linux-gnu %ARM64_TOOLCHAIN%
+New-Item -ItemType SymbolicLink -Path aarch64-linux-gnu -Target $env:ARM64_TOOLCHAIN
+```
+
+```bash
+rustup target add aarch64-unknown-linux-gnu
+```
+
+```bash
+cargo build --target=aarch64-unknown-linux-gnu --release
+```
+
+#### Android
 
 Configure `NDK_HOME` environment variable to point to your Android NDK directory and then create local toolchain replicas in the root project directory using: