[tools] Add arm64 builder MB configs

This is needed to build binaries for arm64 perf testers.

R=machenbach@chromium.org, tmrts@chromium.org

Bug: chromium:920557
Change-Id: I77cea8389b62835a67f221ae26d6682a110ba609
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1605947
Commit-Queue: Sergiy Belozorov <sergiyb@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61613}

infra/mb/mb_config.pyl

@@ -153,6 +153,7 @@
       # Arm64.
       'V8 Android Arm64 - builder': 'release_android_arm64',
       'V8 Android Arm64 - debug builder': 'debug_android_arm64',
+      'V8 Arm64 - builder': 'release_arm64',
       'V8 Linux - arm64 - sim': 'release_simulate_arm64',
       'V8 Linux - arm64 - sim - debug': 'debug_simulate_arm64',
       'V8 Linux - arm64 - sim - nosnap - debug':
@@ -395,6 +396,8 @@
     # Release configs for arm.
     'release_arm': [
       'release_bot', 'arm', 'hard_float'],
+    'release_arm64': [
+      'release_bot', 'arm64', 'hard_float'],
     'release_android_arm': [
       'release_bot', 'arm', 'android', 'minimal_symbols',
       'android_strip_outputs'],