Mips big-endian builders don't like embedded builtins. Disable embedded
builtins on mips for now and defer investigation for when there's time.

Example failures:
https://build.chromium.org/p/client.v8.ports/builders/V8%20Mips%20-%20big%20endian%20-%20nosnap%20-%201/builds/6666
https://build.chromium.org/p/client.v8.ports/builders/V8%20Mips%20-%20big%20endian%20-%20nosnap%20-%202/builds/5188

Two issues: test failures due to big-endianness and mismatches in the
isolate-independence list.

TBR=yangguo@chromium.org

Bug: v8:6666
Change-Id: I73750668c03c132a9bb4d4ae9ed2bafd6d91f816
Reviewed-on: https://chromium-review.googlesource.com/955699Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51848}