[CMake] Be more consistent about naming targets and components in libc++abi

This patch (and commit summary) mirror r279675 by Chris B which was applied
to libc++ but not libc++abi.

Summary:
The point of this patch is to have a consistent convention for naming build,
check and install targets so that the targets can be constructed from the
project name.

This change renames a bunch of CMake components and targets from libcxxabi to
cxxabi. For each renamed target I've added a convenience target that matches
the old target name and depends on the new target. This will preserve function
of the old targets so that the change doesn't break the world. We can evaluate
if it is worth removing the extra targets later.

llvm-svn: 279938