[compiler-rt] Add option to disable libc++ build

Having libc++ checked out doesn't necessarily mean it should be built;
for example, the same source tree might be used for multiple build
configurations, and libc++ might not build in some of those
configurations. Add an option to compiler-rt's build to disable building
libc++. This defaults to ON, so it shouldn't change any existing build
configurations.

Differential Revision: https://reviews.llvm.org/D56479

llvm-svn: 351117