Don't set CC and CXX when running go build on mac

Setting CC and CXX to point to our clang prebuilts on mac fails to build with:
_cgo_export.c:3:10: fatal error: 'stdlib.h' file not found

Skip setting CC and CXX on mac to go back to using the host linker.

Change-Id: I9bab4cda961874c381eacc34397cc67f62fd4730
Test: treehugger
diff --git a/scripts/run-soong-tests-with-go-tools.sh b/scripts/run-soong-tests-with-go-tools.sh
index 9b7d8aa..93c622e 100755
--- a/scripts/run-soong-tests-with-go-tools.sh
+++ b/scripts/run-soong-tests-with-go-tools.sh
@@ -32,11 +32,13 @@
 mkdir -p ${TMPDIR}
 ${GOROOT}/bin/go env
 
-# Building with the race detector enabled uses the host linker, set the
-# path to use the hermetic one.
-CLANG_VERSION=$(build/soong/scripts/get_clang_version.py)
-export CC="${TOP}/prebuilts/clang/host/${OS}-x86/${CLANG_VERSION}/bin/clang"
-export CXX="${TOP}/prebuilts/clang/host/${OS}-x86/${CLANG_VERSION}/bin/clang++"
+if [[ ${OS} = linux ]]; then
+    # Building with the race detector enabled uses the host linker, set the
+    # path to use the hermetic one.
+    CLANG_VERSION=$(build/soong/scripts/get_clang_version.py)
+    export CC="${TOP}/prebuilts/clang/host/${OS}-x86/${CLANG_VERSION}/bin/clang"
+    export CXX="${TOP}/prebuilts/clang/host/${OS}-x86/${CLANG_VERSION}/bin/clang++"
+fi
 
 # androidmk_test.go gets confused if ANDROID_BUILD_TOP is set.
 unset ANDROID_BUILD_TOP