[gdb/testsuite] Fix gdb.base/nostdlib.exp on aarch64

[gdb/testsuite] Fix gdb.base/nostdlib.exp on aarch64

On aarch64-linux, in test-case gdb.base/nostdlib.exp I run into:
...
(gdb) continue^M
Continuing.^M
warning: Temporarily disabling breakpoints for unloaded shared library \
  "/lib/ld-linux-aarch64.so.1"^M
^M
Breakpoint 2, _start () at nostdlib.c:20^M
20      {^M
(gdb) FAIL: $exp: pie=pie: continue to marker
...

This happens as follows:
- the test-case sets a breakpoint on *_start,
- the breakpoint resolves to *_start in the executable,
- the executable is started, and the breakpoint resolves to *_start in the
  dynamic linker,
- execution stops at *_start in the dynamic linker,
- the test-case issues a continue, expecting to continue to the breakpoint on
  marker,
- while continuing, the dynamic linker is reported as unloaded,
- the breakpoint again resolves to *_start in the executable,
- execution stops at *_start in the executable, and
- the test-case concludes that it failed to "continue to marker".

This doesn't happen on x86_64-linux.  There, after the executable is started,
the breakpoint again resolves to *_start in the exec.

This is similar to what happens when printing _start.

On aarch64-linux, we print the _start in the dynamic linker:
...
$ gdb -q -batch outputs/gdb.base/nostdlib/nostdlib-pie \
    -ex "b _start" \
    -ex run \
    -ex "print _start" \
    -ex "info break"
Breakpoint 1 at 0x2bc: file nostdlib.c, line 23.

Breakpoint 1.2, _start () at ../sysdeps/aarch64/dl-start.S:22
22      ENTRY (_start)
$1 = {void (void)} 0xfffff7fd6ac0 <_start>
Num     Type           Disp Enb Address            What
1       breakpoint     keep y   <MULTIPLE>
        breakpoint already hit 1 time
1.1                         y   0x0000aaaaaaaa02bc in _start at nostdlib.c:23
1.2                         y   0x0000fffff7fd6ac0 in _start at dl-start.S:22
...

On x86_64-linux, we print the _start in the exec:
...
Breakpoint 1 at 0x2c5: file nostdlib.c, line 23.

Breakpoint 1.2, 0x00007ffff7fe4f00 in _start () from \
  /lib64/ld-linux-x86-64.so.2
$1 = {void (void)} 0x5555555542c1 <_start>
Num     Type           Disp Enb Address            What
1       breakpoint     keep y   <MULTIPLE>
	breakpoint already hit 1 time
1.1                         y   0x00005555555542c5 in _start at nostdlib.c:23
1.2                         y   0x00007ffff7fe4f00 <_start>
...

The difference may be down to the availability of debug info for the _start in
the dynamic linker.

Finally, the described scenario on aarch64-linux is not deterministic.  The
behavior depends on the dynamic linker being reported as unloaded, which has
been classified as a GLIBC bug, so that might get fixed.

Ideally this test-case would stop at both *_start in the executable and the
dynamic linker, but in absense of a way to specify this reliably (see PR32748),
fix this by making this a temporary breakpoint, ensuring that the breakpoint
will only trigger once.

Approved-by: Kevin Buettner <kevinb@redhat.com>
PR testsuite/32743
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32743