[gdb/testsuite] Fix test-case gdb.threads/attach-stopped.exp on manjaro linux

[gdb/testsuite] Fix test-case gdb.threads/attach-stopped.exp on manjaro linux

When running test-case gdb.threads/attach-stopped.exp on aarch64-linux, using
the manjaro linux distro, I get:
...
 (gdb) thread apply all bt^M
 ^M
 Thread 2 (Thread 0xffff8d8af120 (LWP 278116) "attach-stopped"):^M
 #0  0x0000ffff8d964864 in clock_nanosleep () from /usr/lib/libc.so.6^M
 #1  0x0000ffff8d969cac in nanosleep () from /usr/lib/libc.so.6^M
 #2  0x0000ffff8d969b68 in sleep () from /usr/lib/libc.so.6^M
 #3  0x0000aaaade370828 in func (arg=0x0) at attach-stopped.c:29^M
 #4  0x0000ffff8d930aec in ?? () from /usr/lib/libc.so.6^M
 #5  0x0000ffff8d99a5dc in ?? () from /usr/lib/libc.so.6^M
 ^M
 Thread 1 (Thread 0xffff8db62020 (LWP 278111) "attach-stopped"):^M
 #0  0x0000ffff8d92d2d8 in ?? () from /usr/lib/libc.so.6^M
 #1  0x0000ffff8d9324b8 in ?? () from /usr/lib/libc.so.6^M
 #2  0x0000aaaade37086c in main () at attach-stopped.c:45^M
 (gdb) FAIL: gdb.threads/attach-stopped.exp: threaded: attach2 to stopped bt
...

The problem is that the test-case expects to see start_thread:
...
	gdb_test "thread apply all bt" ".*sleep.*start_thread.*" \
	    "$threadtype: attach2 to stopped bt"
...
but lack of symbols makes that impossible.

Fix this by allowing " in ?? () from " as well.

Tested on aarch64-linux.

PR testsuite/31451
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31451