Adapt to the new libc++/libc++abi update.

C++17 removed a handful of things that are used in the libclang
headers, which are used by the versioner. Enable the flag that
re-exposes these until we have a C++17 compatible libclang.

Add a stub posix_memalign to the linker. libc++abi uses posix_memalign
when allocating exceptions, which the linker does not use.

Test: make checkbuild
Bug: None
Change-Id: I32f9d0591ef99a610f27efed90a5c9fd150f0d3e