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.

llvm-svn: 212672