[libcxx] Support LIBCXX_STATICALLY_LINK_ABI_IN_STATIC_LIBRARY + LIBCXX_ENABLE_ABI_LINKER_SCRIPT

Currently if both LIBCXX_STATICALLY_LINK_ABI_IN_STATIC_LIBRARY (only
applies to static library) and LIBCXX_ENABLE_ABI_LINKER_SCRIPT (only
applies to shared library) are enabled, the former will be silently
ignored. The shared library will use a linker script, while the
static library fails to link libc++abi.a entirely.

This is caused by what appears to be an implementation bug: The
LIBCXX_STATICALLY_LINK_ABI_IN_*_LIBRARY options are declared as
dependent options of LIBCXX_ENABLE_STATIC_ABI_LIBRARY, rather than
simply using it as the default value.

Of course, the combination of
LIBCXX_STATICALLY_LINK_ABI_IN_SHARED_LIBRARY and
LIBCXX_ENABLE_ABI_LINKER_SCRIPT still results in a cmake error,
because these would be conflicting requests for the shared library.

Differential Revision: https://reviews.llvm.org/D134644