In gdb.threads/pthreads.c, handle pthread_attr_setscope ENOTSUP

In gdb.threads/pthreads.c, handle pthread_attr_setscope ENOTSUP

On Cygwin, I see:

 (gdb) PASS: gdb.threads/pthreads.exp: break thread1
 continue
 Continuing.
 pthread_attr_setscope 1: Not supported (134)
 [Thread 3732.0x265c exited with code 1]
 [Thread 3732.0x2834 exited with code 1]
 [Thread 3732.0x2690 exited with code 1]

 Program terminated with signal SIGHUP, Hangup.
 The program no longer exists.
 (gdb) FAIL: gdb.threads/pthreads.exp: Continue to creation of first thread

 ... and then a set of cascading failures.

Fix this by treating ENOTSUP the same way as if PTHREAD_SCOPE_SYSTEM
were not defined.  I.e., ignore ENOTSUP errors, and proceed with
testing.

Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: Iea68ff8b9937570726154f36610c48ef96101871