[libcxxabi] Allow tests to link with static libc++abi/libc++ even if the shared versi...

[libcxxabi] Allow tests to link with static libc++abi/libc++ even if the shared version is present

Summary:
Right now the only way to force libc++abi tests to link with the static version of libc++abi is to set `LIBCXXABI_ENABLE_SHARED` to `OFF`. However, this doesn't work when libc++abi is built as standalone project because of [this](https://github.com/llvm/llvm-project/blob/54c522420347e58aa7bae1892cf5c5672b57c875/libcxxabi/CMakeLists.txt#L503-L519).

This change allows specifying the version of the library for tests to link with.

This is useful for remote testing, for example, with `SSHExecutor`, where we _have_ to link with libc++abi statically.

Two new CMake options are introduced here: `LIBCXXABI_LINK_TESTS_WITH_SHARED_LIBCXXABI` and `LIBCXXABI_LINK_TESTS_WITH_SHARED_LIBCXX`. They can be set to `OFF` to tell the test utility to link tests with the static libraries.

It shouldn't break anything, because the default values of these options are set such that the test utility will behave the same way.

Reviewers: EricWF, mclow.lists, phosek, mehdi_amini, ldionne, jroelofs, bcraig

Subscribers: mgorny, christof, ldionne, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D71894