[gdb/testsuite] Fix gdb.threads/linux-dp.exp

[gdb/testsuite] Fix gdb.threads/linux-dp.exp

On openSUSE Tumbleweed with glibc-debuginfo installed I get:
...
 (gdb) PASS: gdb.threads/linux-dp.exp: continue to breakpoint: thread 5's print
 where^M
 #0  print_philosopher (n=3, left=33 '!', right=33 '!') at linux-dp.c:105^M
 #1  0x0000000000401628 in philosopher (data=0x40537c) at linux-dp.c:148^M
 #2  0x00007ffff7d56b37 in start_thread (arg=<optimized out>) \
                          at pthread_create.c:435^M
 #3  0x00007ffff7ddb640 in clone3 () \
                          at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:81^M
 (gdb) PASS: gdb.threads/linux-dp.exp: first thread-specific breakpoint hit
...
while without debuginfo installed I get instead:
...
 (gdb) PASS: gdb.threads/linux-dp.exp: continue to breakpoint: thread 5's print
 where^M
 #0  print_philosopher (n=3, left=33 '!', right=33 '!') at linux-dp.c:105^M
 #1  0x0000000000401628 in philosopher (data=0x40537c) at linux-dp.c:148^M
 #2  0x00007ffff7d56b37 in start_thread () from /lib64/libc.so.6^M
 #3  0x00007ffff7ddb640 in clone3 () from /lib64/libc.so.6^M
 (gdb) FAIL: gdb.threads/linux-dp.exp: first thread-specific breakpoint hit
...

The problem is that the regexp used:
...
  "\(from .*libpthread\|at pthread_create\|in pthread_create\)"
...
expects the 'from' part to match libpthread, but in glibc 2.34 libpthread has
been merged into libc.

Fix this by updating the regexp.

Tested on x86_64-linux.