GN: Normalize the build directory path BUG=586906 TEST=ln -s /tmp out_tmp && out/gn/gn gen //out_tmp/Release Review URL: https://codereview.chromium.org/1707293002 Cr-Original-Commit-Position: refs/heads/master@{#376288} Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src Cr-Mirrored-Commit: 62b5ce373ce17228a3c06f3e32ff329180e89921