Rename builtins-proxy.cc to builtins-proxy-gen.cc.

Bug: v8:6557, v8:6567
Change-Id: I0e52a0c0c6c9b307c33bb18ec36079bdfd4a89ef
Reviewed-on: https://chromium-review.googlesource.com/565278
Commit-Queue: Maya Lekova <mslekova@google.com>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46511}