Add version, arch, system libs, and targets to Makefile.config

Teach autoconf/configure.ac to AC_SUBST several additional values in
Makefile.config to make them available to Makefile code.  These will
be useful to generate CMake package modules from the Makefile build.

Contributed by Brad King.
Provide CMake package modules in install tree

Teach the Makefile build system to generate and install CMake modules
LLVMConfig.cmake and LLVMConfigVersion.cmake so that applications that
build with CMake can use 'find_package(LLVM)' even when LLVM is not
built with CMake.  These modules tell such applications about available
LLVM libraries and their dependencies.

Run llvm-config to generate the list of libraries and use the results of
llvm-build to generate the library dependencies.  Use sed to perform
substitutions in the LLVMConfig.cmake.in and LLVMConfigVersion.cmake.in
sources that our CMake build system uses.

Teach the Makefile build system to generate the LLVMExports.cmake file
with content similar to that produced by the CMake install(EXPORT)
command.  Extend llvm-build with an option to generate the library
dependencies fragment for this file.

Contributed by Brad King.