[gdb/testsuite] Fix regexps in gdb.base/step-over-syscall.exp

[gdb/testsuite] Fix regexps in gdb.base/step-over-syscall.exp

When running test-case gdb.base/step-over-syscall.exp without glibc debuginfo
installed, I get:
...
(gdb) continue^M
Continuing.^M
^M
Breakpoint 2, 0x00007ffff7d4405e in vfork () from /lib64/libc.so.6^M
(gdb) PASS: gdb.base/step-over-syscall.exp: vfork: displaced=off: \
  continue to vfork (1st time)
...
but with glibc debuginfo installed I get instead:
...
(gdb) continue^M
Continuing.^M
^M
Breakpoint 2, 0x00007ffff7d4405e in __libc_vfork () at \
  ../sysdeps/unix/sysv/linux/x86_64/vfork.S:44^M
44      ENTRY (__vfork)^M
(gdb) FAIL: gdb.base/step-over-syscall.exp: vfork: displaced=off: \
  continue to vfork (1st time)
...

The FAIL is due to a mismatch with regexp:
...
  "Breakpoint \[0-9\]+, (.* in |__libc_|)$syscall \\(\\).*"
...
because it cannot match both ".* in " and the __libc_ prefix.

Fix this by using instead the regexp:
...
  "Breakpoint \[0-9\]+, (.* in )?(__libc_)?$syscall \\(\\).*"
...

Tested on x86_64-linux.