[gdb/testsuite] Use remote_exec chmod instead of remote_spawn

[gdb/testsuite] Use remote_exec chmod instead of remote_spawn

I build gdb using -O2, and ran the testsuite using taskset -c 0, and ran into:
...
(gdb) PASS: gdb.server/connect-with-no-symbol-file.exp: sysroot=: \
  action=delete: setup: adjust sysroot
builtin_spawn gdbserver --once localhost:2385 /connect-with-no-symbol-file^M
/bin/bash: connect-with-no-symbol-file: Permission denied^M
/bin/bash: line 0: exec: connect-with-no-symbol-file: cannot execute: \
  Permission denied^M
During startup program exited with code 126.^M
Exiting^M
target remote localhost:2385^M
`connect-with-no-symbol-file' has disappeared; keeping its symbols.^M
localhost:2385: Connection timed out.^M
(gdb) FAIL: gdb.server/connect-with-no-symbol-file.exp: sysroot=: \
  action=delete: connection to GDBserver succeeded
...

The expected series of events is (skipping disconnect and detach as I don't
think they're relevant to the problem):
- enter scenario "permission"
- cp $exec.bak $exec
- gdbserver start with $exec
- chmod 000 $exec
- connect to gdbserver
- enter scenario "delete"
- cp $exec.bak $exec
- gdbserver start with $exec
- delete $exec
- connect to gdbserver

The problem is that the chmod is executed using remote_spawn:
...
       } elseif { $action == "permission" } {
         remote_spawn target "chmod 000 $target_exec"
       }
...
without waiting on the resulting spawn id, so we're not sure when the
chmod will have effect.

The FAIL we're seeing above is explained by the chmod having effect during the
delete scenario, after the "cp $exec.bak $exec" and before the "gdbserver
start with $exec".

Fix this by using remote_exec instead.

Likewise, fix a similar case in gdb.mi/mi-exec-run.exp.

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29726