Even though we override chromiums UBSan settings in our own BUILD.gn to
include all sanitizers, vptr is still excluded. The reason is that
the vptr sanitizer requires RTTI to be enabled.

The "is_ubsan_vptr" flag will enable RTTI as well as some linker flags
to export libc++abi symbols. Both are required by the vptr sanitizer.

Change-Id: I803ed71781bb3edc824bbe5d1aaa830841b4b304
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1566511Reviewed-by: Simon Zünd <szuend@chromium.org>
Commit-Queue: Sergiy Belozorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60841}