[CMake] Copy over the system debugserver when using LLDB_USE_SYSTEM_DEBUGSERVER

r366433 broke support for the system debugserver. Although the change
was well-intended, it (presumably) unintentionally removed the logic to
copy over the debugserver. As a result, even with
LLDB_USE_SYSTEM_DEBUGSERVER enabled, we ended up building, signing and
using the just-built debugserver.

This patch partially recovers the old behavior: when
LLDB_USE_SYSTEM_DEBUGSERVER is set we don't build debugserver and just
copy over the system one.

Differential revision: https://reviews.llvm.org/D67991

llvm-svn: 372786