[lldb-server] Use Platform plugin corresponding to the host

In ee232506 I moved UnixSignal
initialization from lldbTarget to the various platform plugins. This
inadvertently broke lldb-server because lldb-server doesn't use
Platform plugins. lldb-server still needs to be able to create a
UnixSignals object for the host platform so we can add the relevant
platform plugin to lldb-server to make sure we always have a
HostPlatform.

Differential Revision: https://reviews.llvm.org/D146668