[CMake] Add LLVM_BUILD_32_BITS to LLVMConfig.cmake

Previously out-of-tree passes could detect if LLVM was built with
LLVM_BUILD_32_BITS by looking for -m32 in LLVM_DEFINITIONS, but as of r271871
it no longer appears there. Resolve this by instead emitting LLVM_BUILD_32_BITS
in LLVMConfig so it can be checked for directly.

Differential Revision: http://reviews.llvm.org/D21434

llvm-svn: 274351