[cmake] Teach the llvm-config program to respect LLVM_LIBDIR_SUFFIX.

[cmake] Teach the llvm-config program to respect LLVM_LIBDIR_SUFFIX.

For this to work, we have to encode it in the build variables and use it
from llvm-config.cpp. I've tried to do this reasonably cleanly, but the
code for llvm-config.cpp is pretty strange. However, with this,
llvm-config stops giving the wrong answer when using LLVM_LIBDIR_SUFFIX.

Note that the configure+make build just sets this to an empty string as
that build system has zero support for multilib of any form. I'm not
planning to add support there either, but this should leave a path for
anyone that wanted to.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224921 91177308-0d34-0410-b5e6-96231b3b80d8