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/toolchain.go b/cc/config/toolchain.go
index 995c8c6..8fc4a21 100644
--- a/cc/config/toolchain.go
+++ b/cc/config/toolchain.go
@@ -16,6 +16,7 @@
 
 import (
 	"fmt"
+	"path/filepath"
 
 	"android/soong/android"
 )
@@ -47,6 +48,7 @@
 	GccTriple() string
 	// GccVersion should return a real value, not a ninja reference
 	GccVersion() string
+	ToolPath() string
 
 	ToolchainCflags() string
 	ToolchainLdflags() string
@@ -145,6 +147,10 @@
 	return true
 }
 
+func (t toolchainBase) ToolPath() string {
+	return ""
+}
+
 type toolchain64Bit struct {
 	toolchainBase
 }
@@ -216,3 +222,10 @@
 	}
 	return "libclang_rt.ubsan_standalone-" + arch + "-android.so"
 }
+
+func ToolPath(t Toolchain) string {
+	if p := t.ToolPath(); p != "" {
+		return p
+	}
+	return filepath.Join(t.GccRoot(), t.GccTriple(), "bin")
+}