Factor out common URL handling code in PlatformRemoteGDBServer

Summary:
this also gets rid of a compiler warning in release builds by using a dynamically allocated
buffer. Therefore, a size assertion is not necessary (and probably should have been an error in
the first place).

Reviewers: tberghammer

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D7751

llvm-svn: 229878