[libc++][CMake] Make sure the benchmarks link against libc++abi

The refactoring in r357811 made it so that we didn't add the ABI library
to the list of LIBCXX_LIBRARIES. As a result, benchmarks didn't link to
the ABI library and were missing symbols. This broke the build bots.

As a drive-by fix, we also provide the SHARED ABI library to the linker
script instead of the STATIC ABI library.

This couldn't be discovered on Apple platforms because libc++.dylib
re-exports libc++abi.dylib symbols there.

llvm-svn: 357818