[build] Stop using sysroot on legacy gyp bots

The updated stretch sysroot doesn't work anymore with gyp.

Bug: chromium:788679
Change-Id: I26e196f29eeda3914399655a156b39ba26c9e988
Reviewed-on: https://chromium-review.googlesource.com/789940Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49636}

infra/mb/mb_config.pyl

@@ -530,12 +530,12 @@
 
     # Gyp release configs for mips.
     'gyp_release_mips_no_snap_no_i18n': [
-      'gyp', 'release', 'mips', 'crosscompile', 'static', 'v8_no_i18n',
-      'v8_snapshot_none'],
+      'gyp', 'release', 'mips', 'crosscompile', 'no_sysroot', 'static',
+      'v8_no_i18n', 'v8_snapshot_none'],
 
     # Gyp release configs for x64.
     'gyp_release_x64': [
-      'gyp', 'release_bot', 'x64', 'swarming'],
+      'gyp', 'release_bot', 'x64', 'no_sysroot', 'swarming'],
   },
 
   'mixins': {
@@ -675,6 +675,11 @@
       'gn_args': 'is_clang=false',
     },
 
+    'no_sysroot': {
+      'gn_args': 'use_sysroot=false',
+      'gyp_defines': 'use_sysroot=0',
+    },
+
     'release': {
       'gn_args': 'is_debug=false',
     },