[gdb/tdep] Handle memory error in s390_linux_get_syscall_number

[gdb/tdep] Handle memory error in s390_linux_get_syscall_number

In s390_linux_get_syscall_number, we use read_memory_unsigned_integer, which
can throw a memory error.

According to the function comment though, it should return -1 on error:
...
/* Retrieve the syscall number at a ptrace syscall-stop.  Return -1
   upon error. */
...

Catch the memory error by using safe_read_memory_unsigned_integer instead,
similar to how that was fixed for arm in commit eb42bb14895 ("[gdb/tdep] Fix
catching syscall execve exit for arm").

Approved-By: Ulrich Weigand <uweigand@de.ibm.com>