[lldb] [gdb-remote client] Avoid zero padding PID/TID in H packet

Change SetCurrentThread*() logic not to include the zero padding
in PID/TID that was a side effect of 02ef0f5a.  This should fix
problems caused by sending 64-bit integers to 32-bit servers.  Reported
by Ted Woodward.

Differential Revision: https://reviews.llvm.org/D106832