Always use the standard OSX SDK tools

Instead of whatever happens to be in PATH.

Test: port install gcc5; attempt to build host binaries
Change-Id: If4af32596edd6fd3e459c3d574ab0ff495f236ff
diff --git a/cc/config/x86_darwin_host.go b/cc/config/x86_darwin_host.go
index 18acef8..b6b08fe 100644
--- a/cc/config/x86_darwin_host.go
+++ b/cc/config/x86_darwin_host.go
@@ -17,6 +17,7 @@
 import (
 	"fmt"
 	"os/exec"
+	"path/filepath"
 	"strings"
 
 	"android/soong/android"
@@ -131,6 +132,11 @@
 		return strings.TrimSpace(string(bytes)), err
 	})
 
+	pctx.VariableFunc("MacToolPath", func(config interface{}) (string, error) {
+		bytes, err := exec.Command("xcrun", "--find", "ld").Output()
+		return filepath.Dir(strings.TrimSpace(string(bytes))), err
+	})
+
 	pctx.StaticVariable("DarwinGccVersion", darwinGccVersion)
 	pctx.SourcePathVariable("DarwinGccRoot",
 		"prebuilts/gcc/${HostPrebuiltTag}/host/i686-apple-darwin-${DarwinGccVersion}")
@@ -276,6 +282,10 @@
 	return false
 }
 
+func (t *toolchainDarwin) ToolPath() string {
+	return "${config.MacToolPath}"
+}
+
 var toolchainDarwinX86Singleton Toolchain = &toolchainDarwinX86{}
 var toolchainDarwinX8664Singleton Toolchain = &toolchainDarwinX8664{}