Do not install llvm-config-host for cross-builds of clang. rdar://11317847

My previous change to install llvm-config-host for cross-builds resulted
in that file being installed even when the normal llvm-config was not
installed, e.g., when building the install-clang target.  Daniel suggested
this alternative, which solves the immediate problem and also avoids the gunk
in the top-level makefile.

llvm-svn: 156448