update.py: Download platform-appropriate prebuilt clang

Also tweak the newline at the end of the stamp file to be
compatible with update.sh. This way, if either downloads
clang the other will consider it up-to-date.

BUG=494442
NOTRY=true

Review URL: https://codereview.chromium.org/1468243002

Cr-Commit-Position: refs/heads/master@{#361236}

tools/clang/scripts/update.py

@@ -97,7 +97,7 @@ def ReadStampFile():
   """Return the contents of the stamp file, or '' if it doesn't exist."""
   try:
     with open(STAMP_FILE, 'r') as f:
-      return f.read()
+      return f.read().rstrip()
   except IOError:
     return ''
 
@@ -108,6 +108,7 @@ def WriteStampFile(s):
     os.makedirs(os.path.dirname(STAMP_FILE))
   with open(STAMP_FILE, 'w') as f:
     f.write(s)
+    f.write('\n')
 
 
 def GetSvnRevision(svn_repo):
@@ -281,7 +282,13 @@ def UpdateClang(args):
 
   if not args.force_local_build:
     cds_file = "clang-%s.tgz" %  PACKAGE_VERSION
-    cds_full_url = CDS_URL + '/Win/' + cds_file
+    if sys.platform == 'win32':
+      cds_full_url = CDS_URL + '/Win/' + cds_file
+    elif sys.platform == 'darwin':
+      cds_full_url = CDS_URL + '/Mac/' + cds_file
+    else:
+      assert sys.platform.startswith('linux')
+      cds_full_url = CDS_URL + '/Linux_x64/' + cds_file
 
     # Check if there's a prebuilt binary and if so just fetch that. That's
     # faster, and goma relies on having matching binary hashes on client and