[libcxxabi] Build both static and shared versions of libc++abi by default.

This patch builds both static and shared versions of libc++abi by default. It adds/repurposes the following cmake options:
* `LIBCXXABI_ENABLE_SHARED`: Enable/disable building the shared library. (Previously using `OFF` would build the static library instead)
* `LIBCXXABI_ENABLE_STATIC`: Enable/disable building the static library.

This patch also re-purposes the CMake target `cxxabi` to be a meta-target for `cxxabi_shared` and `cxxabi_static`. This could potentially break other builds that depend on `cxxabi` being a library target. We will need to apply a patch to libc++'s CMake before committing this change. 

Running the tests is still only supported when the shared version is built. Support for running the tests against the static library will come in another patch.

Reviewers: jroelofs, mclow.lists, danalbert, compnerd

Reviewed By: danalbert, compnerd

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D8013

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@231075 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed