Skip TestGdbserverPort.test on Windows

lldb/cmake/modules/LLDBConfig.cmake does not build lldb-server on Windows:
if (CMAKE_SYSTEM_NAME MATCHES "Android|Darwin|FreeBSD|Linux|NetBSD")
    set(LLDB_CAN_USE_LLDB_SERVER 1)

Also do not append 'platform' parameter twice - although that was quietly
ignored.

llvm-svn: 355579