Roll buildtools.

Picks up:
gn roll https://codereview.chromium.org/1547563004/
libcxx/libcxxabi roll: https://codereview.chromium.org/1559483002/
Revert gn roll: https://codereview.chromium.org/1559513002/
Fix libcxx build: https://codereview.chromium.org/1554723003/

BUG=573250
R=thestig@chromium.org
TBR=agrieve

Review URL: https://codereview.chromium.org/1551913002 .

Cr-Commit-Position: refs/heads/master@{#367180}

DEPS

@@ -55,7 +55,7 @@ vars = {
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling build tools
   # and whatever else without interference from each other.
-  'buildtools_revision': 'fee7f1e849f59c3fd7bb7b2cacf876edca0572f3',
+  'buildtools_revision': '71d2e70bfd5ae0a2388abc49ac064c5d35e3c2df',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling PDFium
   # and whatever else without interference from each other.

tools/checklicenses/checklicenses.py

@@ -121,6 +121,14 @@ PATH_SPECIFIC_WHITELISTED_LICENSES = {
         # http://llvm.org/bugs/show_bug.cgi?id=18291
         'UNKNOWN',
     ],
+    # http://llvm.org/bugs/show_bug.cgi?id=25976
+    'buildtools/third_party/libc++/trunk/src/include/atomic_support.h': [
+      'UNKNOWN'
+    ],
+    'buildtools/third_party/libc++/trunk/utils/gen_link_script': [ 'UNKNOWN' ],
+    'buildtools/third_party/libc++/trunk/utils/not': [ 'UNKNOWN' ],
+    'buildtools/third_party/libc++/trunk/utils/sym_check': [ 'UNKNOWN' ],
+    'buildtools/third_party/libc++abi/trunk/test': [ 'UNKNOWN' ],
 
     'chrome/common/extensions/docs/examples': [  # http://crbug.com/98092
         'UNKNOWN',