[Host] Respect LLVM_LIBDIR_SUFFIX when looking for LLDB plugins on Linux

Fix the Linux plugin lookup path to include appropriate libdir suffix
for the system. To accomplish this, store the value of
LLVM_LIBDIR_SUFFIX in lldb/Host/Config.h as LLDB_LIBDIR_SUFFIX,
and use this variable when defining the plugin path.

Differential Revision: https://reviews.llvm.org/D42317

llvm-svn: 323673