[CMake] Set per-runtime library directory suffix in runtimes build

Do not use LLVM_RUNTIMES_LIBDIR_SUFFIX variable which is an internal
variable used by the runtimes build from individual runtimes, instead
set per-runtime librarhy directory suffix variable which is necessary
for the sanitized runtimes build to install libraries into correct
location.

Differential Revision: https://reviews.llvm.org/D49121

llvm-svn: 336713