Disable _ExtInt by default

Since the _ExtInt type got into the repo, we've discovered that the ABI
implications weren't completely understood. The other architectures are
going to be audited (see D79118), however downstream targets aren't
going to benefit from this audit.

This patch disables the _ExtInt type by default and makes the
target-info an opt-in.  As it is audited, I'll re-enable these for all
of our default targets.