[gdb/testsuite] Fix gdb.server/sysroot.exp for remote target

Fix test-case gdb.server/sysroot.exp with target board
remote-gdbserver-on-localhost, by:
- using gdb_remote_download, and
- disabling the "local" scenario for remote host.

Tested on x86_64-linux.

gdb/testsuite/gdb.server/sysroot.exp

@@ -27,9 +27,17 @@ if {[build_executable "failed to prepare" $testfile $srcfile "additional_flags=-
     return -1
 }
 
+set target_binfile [gdb_remote_download target $binfile]
+
 # Run once with sysroot set to the local filesystem and once set to the remote
 # target.
-foreach_with_prefix sysroot { "local" "remote" } {
+set modes {}
+if { ![is_remote target] } {
+    lappend modes "local"
+}
+lappend modes "remote"
+
+foreach_with_prefix sysroot $modes {
     global srcdir
     global subdir
     global binfile
@@ -40,7 +48,7 @@ foreach_with_prefix sysroot { "local" "remote" } {
 	set timeout_factor 1
     } else {
 	set sysroot_command "target:"
-	set reading_symbols "Reading $binfile from remote target..."
+	set reading_symbols "Reading .*$target_binfile from remote target..."
 	set timeout_factor 5
     }
 
@@ -55,7 +63,7 @@ foreach_with_prefix sysroot { "local" "remote" } {
 	gdb_test "disconnect" ".*"
 
 	# Start GDBserver.
-	set res [gdbserver_start "" $binfile]
+	set res [gdbserver_start "" $target_binfile]
 	set gdbserver_protocol [lindex $res 0]
 	set gdbserver_gdbport [lindex $res 1]