Point to the gcc buildbot builder.

Turns out I was wrong: the gcc builder must be migrated to LUCI before
we can add it to the LUCI version of chromium.linux. For now, point to
the builder that's actually working.

Bug: chromium:776284
Change-Id: Ifffe4eaa634aad6ec95360ced04047b61c816315
Reviewed-on: https://chromium-review.googlesource.com/995492Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548178}

infra/config/global/luci-milo-dev.cfg

@@ -403,7 +403,7 @@ consoles: {
     short_name: "tst"
   }
   builders: {
-    name: "buildbucket/luci.chromium.ci/linux-gcc-rel"
+    name: "buildbot/chromium.linux/linux-gcc-rel"
     category: "chromium.linux|release"
     short_name: "gcc"
   }

infra/config/global/luci-milo.cfg

@@ -413,7 +413,7 @@ consoles: {
     short_name: "tst"
   }
   builders: {
-    name: "buildbucket/luci.chromium.ci/linux-gcc-rel"
+    name: "buildbot/chromium.linux/linux-gcc-rel"
     category: "chromium.linux|release"
     short_name: "gcc"
   }