Add lit configs for libcxxabi tests.

This makes running libcxxabi tests on Linux _much_ easier.
Adds a check-libcxxabi target to cmake.

Also defaults to building a dynamic libc++abi. This is so that the
default options still test the libc++abi that is being built. There are
two problems with testing a static libc++abi. In the case of a
standalone build, the tests will link the system's libc++, which might
not have been built against our libc++abi. In the case of an in tree
build, libc++ will prefer a dynamic libc++abi from the system over a
static libc++abi from the output directory.


git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@212672 91177308-0d34-0410-b5e6-96231b3b80d8