[gdb/testsuite] Fix typo in gdb.base/catch-syscall.exp

On aarch64-linux with a gdb build without libexpat, I run into:
...
(gdb) PASS: gdb.base/catch-syscall.exp: determine pipe syscall: \
  catch syscall 59
continue
Continuing.

Catchpoint 5 (call to syscall 59), 0x0000fffff7e04578 in pipe () from \
  /lib64/libc.so.6
(gdb) FAIL: gdb.base/catch-syscall.exp: determine pipe syscall: continue
...

In the test-case, this pattern handles either the syscall name or number for
the pipe syscall:
...
  -re -wrap "Catchpoint $decimal \\(call to syscall (pipe|$SYS_pipe)\\).*" {
...
but the pattern for the pipe2 syscall mistakenly uses SYS_pipe instead of
SYS_pipe2:
...
  -re -wrap "Catchpoint $decimal \\(call to syscall (pipe2|$SYS_pipe)\\).*" {
...
and consequently doesn't handle the pipe2 syscall number.

Fix the typo by using SYS_pipe2 instead.

Tested on aarch64-linux.