gdb/testsuite: set sysroot in gdb.server/stop-reply-no-thread-multi.exp

I get some random timeouts in this test due to big debug info taking a
lot of time to read through gdbserver.  When host and target are on the
same machine, clear the sysroot parameter so that GDB reads the files
from the local file system, as we already do in many tests.

I agree with what Pedro says here:

  https://sourceware.org/pipermail/gdb-patches/2019-March/156568.html

that if this is bad for us, it's also bad for users, so we should be
fixing the slowness instead.  But so far nobody seems to be working on
it, and the testsuite timeouts are getting in the way, so I think this
"set sysroot" is a net positive for now.

Without this patch, the test takes over 2 minutes to run (most of it
"downloading" libc debug info), with it it takes 10 seconds.

gdb/testsuite/ChangeLog:

	* gdb.server/stop-reply-no-thread-multi.exp: Clear sysroot when
	host and target are local.

Change-Id: Ieb6304f0e56b4575af450913de4210c667c6bf7b