gdb: testsuite: use gdb_attach to fix attach-pie-noexec.exp

gdb: testsuite: use gdb_attach to fix attach-pie-noexec.exp

If /proc/sys/kernel/yama/ptrace_scope is 1, when execute the following
command without superuser:

  make check-gdb TESTS="gdb.base/attach-pie-noexec.exp"

we can see the following messages in gdb/testsuite/gdb.log:

  (gdb) attach 6500
  Attaching to process 6500
  ptrace: Operation not permitted.
  (gdb) PASS: gdb.base/attach-pie-noexec.exp: attach

It is obviously wrong, the expected result should be UNSUPPORTED in such
a case.

With this patch, we can see "Operation not permitted" in the log info,
and then we can do the following processes to test:
(1) set ptrace_scope as 0
    $ echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope
    $ make check-gdb TESTS="gdb.base/attach-pie-noexec.exp"
(2) use sudo
    $ sudo make check-gdb TESTS="gdb.base/attach-pie-noexec.exp"

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>