[libcxx] Add an explicit option to build against system-libcxxabi

Add an explicit LIBCXX_CXX_ABI=system-libcxxabi option for linking to
system-installed libc++abi. This fixes the ability to link against one
when building libcxx via the runtimes build, as otherwise the build
system insists on linking into in-tree targets.

Differential Revision: https://reviews.llvm.org/D119539