[libc++/abi] Provide an option to turn on forgiving dynamic_cast when building libc...

[libc++/abi] Provide an option to turn on forgiving dynamic_cast when building libc++abi

Instead of the ad-hoc #define _LIBCXX_DYNAMIC_FALLBACK, provide an option
to enable the setting when building libc++abi. Also use the occasion to
rename the option to something slightly more descriptive.

Note that in the future, it would be great to simply remove this option
altogether. However, in the meantime, it seems better to have it be an
official option than something ad-hoc.