gdb/testsuite: use "set sysroot" in gdb.multi/multi-target.exp.tcl

gdb/testsuite: use "set sysroot" in gdb.multi/multi-target.exp.tcl

The multi-target tests involve some inferiors using remote targets.  By
default, GDB uses target: as the sysroot, which makes it read loaded
libraries and their debug info through GDBserver.  This makes the tests
run slower than necessary.

Pass `-ex "set sysroot"` when launching GDB in these tests, so that GDB
always reads from its local file system.

On a system where I don't have debug info for libc, that reduces run
time for

    $ make check TESTS="gdb.multi/multi-target-*.exp"

from 1:15 to 0:45.

On this other system where debug info is installed though, it reduces it
from 13:00 to 1:45.

gdb/testsuite/ChangeLog:

	* gdb.multi/multi-target.exp.tcl (setup): Add "set sysroot" to
	GDBFLAGS.

Change-Id: I9d24f3def843472d35dfb5667c12d70ae1d7e984