[libcxxabi] Automatically use static libunwind when required

If we attempt to use unwind_shared when LIBUNWIND_ENABLE_SHARED is OFF,
we'll get link errors. LIBCXXABI_STATICALLY_LINK_UNWINDER_IN_SHARED_LIBRARY
avoids that, but it seems redundant to have to specify it manually.
Automatically switch libunwind statically when its shared build is
disabled, which also matches compiler-rt [1].

[1] https://github.com/llvm/llvm-project/blob/71bfec762bd970e7834f58c158ddc15f93402d7a/compiler-rt/CMakeLists.txt#L238-L240

Reviewed By: #libc_abi, phosek

Differential Revision: https://reviews.llvm.org/D158789