gdb: sim: handle target sysroot prefix

gdb: sim: handle target sysroot prefix

The default gdb sysroot now sets itself to "target:".  This works for
most remote targets, but when using the simulator, this causes problems
as the sim will attempt to search for that path.

Update the remote-sim logic to skip this leading prefix when it is found
so that the sysroot isn't passed in as an invalid value.