Teach LLVM-Config to use logical target names (2/2)

The module still needs to collect the list of all available libraries
in order to satisfy the 'all' component.  Provide this in the package
configuration file, 'LLVMConfig.cmake', as a LLVM_AVAILABLE_LIBS
variable.  (A variable is scoped better than a global property.)
Since this won't be set for our own build, fall back to looking up the
LLVM_LIBS property to get the value when it is not set.

Contributed by Brad King.

llvm-svn: 201853