RNBRemote.cpp, RNBRemote.h: Add a new qGetPid packet which returns

the pid of the process currently being debugged by debugserer in
hex, or 0 if unavailable.

This is effectively the same as the qC packet but that packet is
not clear in either its documentation or implementation (in gdb et al)
as to whether it is intended to return a pid or a thread id.  qGetPid
is unambiguous.

If qGetPid is unimplemented in the remote debugserver, the debugger may
try qC and see what kind of value is returned..

llvm-svn: 136055