[libcxx][cmake] Remove libatomic temporarily from CMAKE_REQUIRED_LIBRARIES when configuring

When libcxx is built in tree for a host which requires libatomic, LLVM's
configuration steps will determine it is required and add it to
CMAKE_REQUIRED_LIBRARIES. When libcxx is later configured, it tests if it
has C++ atomics without libatomic. The test erroneously passes as libatomic
is already part of the set of required libraries.

In turn, a number of the atomic tests will fail as they require libatomic
but the test suite is configured not to use libatomic.

Address this by always dropping libatomic from the set of required libraries
before determining if LIBCXX_HAVE_CXX_ATOMICS_WITHOUT_LIB is true,
then restoring the set of required libraries.

Reviewers: EricWF

Differential Revision: https://reviews.llvm.org/D43509

llvm-svn: 329167