[llvm] [cmake] Set EXCLUDE_FROM_ALL on gtest and TestingSupport

Exclude building googletest and LLVMTestingSupport libraries from
the `all` target.  If unittests are being built, these libraries will
be built as a dependency anyway.  If they are not being built, building
them makes little sense as they are not installed or used otherwise.

This will also make standalone builds of other projects easier, as it
makes it possible to include these directories without having to cover
them with additional conditions to prevent them from being built
unconditionally.

Differential Revision: https://reviews.llvm.org/D137035