feat: add native Linux ARM64 build target (linuxarm64) (#439)

generateLibs' Linux target has always been constructed via
newDefaultTarget(Os.Linux, Bitness._64), which defaults to
Architecture.x86 regardless of host architecture. That's the root
cause behind #105: even on a native aarch64 host, the task emits
x86-only compiler flags (-mfpmath=sse -msse -m64), which a real
aarch64 g++/gcc rejects outright.

gdx-jnigen 2.5.2 (the library this build script sits on top of)
already ships a correct Architecture.ARM Linux target
(aarch64-linux-gnu- prefix, -fPIC, no SSE flags) — this task just
never selects it.

Add an opt-in `linuxarm64` env (mirroring the existing `macosarm64`
convention) that builds via that target instead, refactoring the
existing x86 Linux target into a shared createLinuxTarget() helper
(mirroring createMacTarget()) so both paths stay in sync. libName is
pinned to libimgui-java64.so on both architectures for consistency,
matching how the mac target already does this for x86_64/arm64.

Author: greenjon

buildSrc/src/main/groovy/tool/generator/GenerateLibs.groovy

@@ -32,6 +32,7 @@ class GenerateLibs extends DefaultTask {
     private final String[] buildEnvs = System.getProperty('envs')?.split(',')
     private final boolean forWindows = buildEnvs?.contains('windows')
     private final boolean forLinux = buildEnvs?.contains('linux')
+    private final boolean forLinuxArm64 = buildEnvs?.contains('linuxarm64')
     private final boolean forMac = buildEnvs?.contains('macos')
     private final boolean forMacArm64 = buildEnvs?.contains('macosarm64')
 
@@ -120,10 +121,11 @@ class GenerateLibs extends DefaultTask {
         }
 
         if (forLinux) {
-            def linux64 = BuildTarget.newDefaultTarget(Os.Linux, Architecture.Bitness._64)
-            requireCpp17(linux64)
-            addFreeTypeIfEnabled(linux64)
-            buildTargets += linux64
+            buildTargets += createLinuxTarget(Architecture.x86)
+        }
+
+        if (forLinuxArm64) {
+            buildTargets += createLinuxTarget(Architecture.ARM)
         }
 
         if (forMac) {
@@ -145,6 +147,8 @@ class GenerateLibs extends DefaultTask {
             BuildExecutor.executeAnt(jniDir + '/build-windows64.xml', commonParams)
         if (forLinux)
             BuildExecutor.executeAnt(jniDir + '/build-linux64.xml', commonParams)
+        if (forLinuxArm64)
+            BuildExecutor.executeAnt(jniDir + '/build-linuxarm64.xml', commonParams)
         if (forMac)
             BuildExecutor.executeAnt(jniDir + '/build-macosx64.xml', commonParams)
         if (forMacArm64)
@@ -156,6 +160,8 @@ class GenerateLibs extends DefaultTask {
             checkLibExist("windows64/imgui-java64.dll")
         if (forLinux)
             checkLibExist("linux64/libimgui-java64.so")
+        if (forLinuxArm64)
+            checkLibExist("linuxarm64/libimgui-java64.so")
         if (forMac)
             checkLibExist("macosx64/libimgui-java64.dylib")
         if (forMacArm64)
@@ -170,6 +176,21 @@ class GenerateLibs extends DefaultTask {
         }
     }
 
+    BuildTarget createLinuxTarget(Architecture arch) {
+        def linuxTarget = BuildTarget.newDefaultTarget(Os.Linux, Architecture.Bitness._64, arch)
+        linuxTarget.libName = "libimgui-java64.so" // Lib for arm64 will be named the same for consistency.
+        if (arch == Architecture.ARM && System.getProperty('os.arch') in ['aarch64', 'arm64']) {
+            // gdx-jnigen defaults ARM Linux targets to the aarch64-linux-gnu- cross-compiler
+            // prefix. When already running natively on an aarch64 host (e.g. GitHub's
+            // ubuntu-24.04-arm runners) that prefix isn't needed, and the cross toolchain
+            // package providing it usually isn't installed there anyway.
+            linuxTarget.compilerPrefix = ''
+        }
+        requireCpp17(linuxTarget)
+        addFreeTypeIfEnabled(linuxTarget)
+        return linuxTarget
+    }
+
     BuildTarget createMacTarget(Architecture arch) {
         def minMacOsVersion = '10.15'
         def macTarget = BuildTarget.newDefaultTarget(Os.MacOsX, Architecture.Bitness._64, arch)