[libc++abi] Add temporary workaround to unblock Chrome

Chrome rolls libc++ and libc++abi as separate projects. As a result, they
may not always be updated in lockstep, and this can lead to build failures
when mixing libc++ that doesn't have <__thread/support.h> with libc++abi
that requires it.

This patch adds a workaround to make libc++abi work with both versions.
While Chrome's setup is not supported, this workaround will allow them
to go back to green and do the required work needed to roll libc++ and
libc++abi in lockstep. This workaround will be short-lived -- I have a
reminder to go back and remove it by EOW.

GitOrigin-RevId: 372f7dd48f0165c6a7cdc819516132c617ef6f45
5 files changed