feat(gemma-iree): add macosArm64 target

Adds Apple-Silicon support to the on-device gemma runtime, mirroring
:llm-runtime:kgemma (which already ships macos_arm64). The IreeRuntime +
GemmaDecoder sources live in nativeMain, so macosArm64Main picks them up with no
new code — posix popen/fgets/pclose + kotlinx-io + the GGUF tokenizer are all
available on macOS. Lets downstream KMP consumers (e.g. gemma-tool-calling) add
a macosArm64 target.

Note: macOS compile/link runs on the macOS publish/CI runner, not Linux CI
(which skips mac link tasks, as it already does for kgemma).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: michalharakal

llm-runtime/gemma-iree/build.gradle.kts

@@ -15,6 +15,7 @@ kotlin {
     jvm()
     linuxX64()
     linuxArm64()
+    macosArm64() // Apple Silicon host dev (mirrors :llm-runtime:kgemma); uses the same nativeMain sources
 
     sourceSets {
         commonTest.dependencies {