gollvm: add /usr/{lib,lib32} to toolchain paths for arch linux

this uses Distro to make adding /usr/ path only happen if build system is running Arch Linux

Change-Id: I0b91b717a42ed643ad049ea206fa4a9f3c66df9a
Reviewed-on: https://go-review.googlesource.com/c/gollvm/+/399876
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
diff --git a/driver/LinuxToolChain.cpp b/driver/LinuxToolChain.cpp
index 22a6120..73037f1 100644
--- a/driver/LinuxToolChain.cpp
+++ b/driver/LinuxToolChain.cpp
@@ -73,6 +73,8 @@
                                       "/../" + ftrip).str());
   addIfPathExists(fpaths, llvm::Twine(osLibDir).str());
   addIfPathExists(fpaths, llvm::Twine(osLibDir + "/" + ftrip).str());
+  if (this->distro_ == distro::DistroArchLinux)
+    addIfPathExists(fpaths, llvm::Twine("/usr/" + osLibDir).str());
 
   // Include program and file paths in verbose output.
   if (driver.args().hasArg(gollvm::options::OPT_v)) {