[cmake/multilib] Teach llgo to respect the LLVM_LIBDIR_SUFFIX variable for

multilib build and installs.

Summary:
This requires introducing a generated header to encapsulate the
LLVM_LIBDIR_SUFFIX value from the build system and push it into the go
code. From there, I've adjusted the gllgo code to systematically use
this rather than a raw "lib". This requires some awkwardness as one of
the flags *must* be "lib"-relative for compatibility with how gccgo
works. For that flag, we use ".." to back up a directory and then go
into the proper lib directory.

Differential Revision: http://reviews.llvm.org/D6795

llvm-svn: 224964