[gdb/testsuite] Fix gdb.base/catch-syscall.exp with --with-expat=no

[gdb/testsuite] Fix gdb.base/catch-syscall.exp with --with-expat=no

When doing a gdb build with --with-expat=no, I run into:
...
(gdb) PASS: gdb.base/catch-syscall.exp: determine pipe syscall: \
  continue to breakpoint: before pipe call
catch syscall pipe^M
Unknown syscall name 'pipe'.^M
(gdb) PASS: gdb.base/catch-syscall.exp: determine pipe syscall: \
  catch syscall pipe
catch syscall pipe2^M
Unknown syscall name 'pipe2'.^M
(gdb) PASS: gdb.base/catch-syscall.exp: determine pipe syscall: \
  catch syscall pipe2
continue^M
Continuing.^M
[Detaching after vfork from child process 18538]^M
[Inferior 1 (process 18537) exited normally]^M
(gdb) FAIL: gdb.base/catch-syscall.exp: determine pipe syscall: continue
...

This is a regression since recent commit 5463a15c18b ("[gdb/testsuite] Handle
pipe2 syscall in gdb.base/catch-syscall.exp").

Fix this by using pipe/pipe2 syscall numbers instead.

Tested on x86_64-linux.