Use the correct ninja binary for the host in aninja

Change-Id: I79eb7fb397e9ae623a1cc2e67a472881767656f6
Test: `aninja -h` on Linux & Mac
diff --git a/bin/aninja b/bin/aninja
index cceb794..5acb968 100755
--- a/bin/aninja
+++ b/bin/aninja
@@ -20,6 +20,19 @@
 require_top
 require_lunch
 
+case $(uname -s) in
+    Darwin)
+        host_arch=darwin-x86
+        ;;
+    Linux)
+        host_arch=linux-x86
+        ;;
+    *)
+        >&2 echo Unknown host $(uname -s)
+        exit 1
+        ;;
+esac
+
 cd $(gettop)
-prebuilts/build-tools/linux-x86/bin/ninja -f out/combined-${TARGET_PRODUCT}.ninja "$@"
+prebuilts/build-tools/${host_arch}/bin/ninja -f out/combined-${TARGET_PRODUCT}.ninja "$@"