[runtimes] Unify exception and RTTI configuration for the runtimes

[ldionne]

This patch introduces a unified mechanism for configuring whether exceptions and RTTI are enabled across the runtimes. It does that by introducing new options named RUNTIMES_ENABLE_{EXCEPTIONS,RTTI} and providing a CMake interface target that can be used by various runtime targets (cxx_shared, cxx_static, etc) to get the relevant flags.

This patch aims to create a precedent and an example that can be followed to unify other similar configuration options across the runtimes.

There are still the following open questions:

• Does it make sense at all to have a unified option for all the runtimes? For example, is it desirable to be able to build libc++ without exception support, but libc++abi with exception support?
• How should we deal with options where one of the runtimes does not allow configuring an option? For example, libunwind currently doesn't allow configuring whether exceptions are enabled: they are always enabled. If libunwind links against the interface target for language-level flags and RUNTIMES_ENABLE_EXCEPTIONS=OFF is used, that won't work.
• Where should we document these new unified options? Should we create a documentation page for all of the runtimes?