[CMake] Move debugserver options to separate debugserverConfig.cmake

Summary:
One place for debugserver options, analog to LLDBConfig for LLDB options (see D55317). It was discussed in earlier reviews already, e.g. D55013.

Reviewers: JDevlieghere, aprantl, xiaobai

Reviewed By: aprantl, xiaobai

Subscribers: mgorny, lldb-commits, #lldb

Differential Revision: https://reviews.llvm.org/D55320

llvm-svn: 350390