[gdb/tdep] Use symbolic constants in s390_prologue_frame_unwind_cache

[gdb/tdep] Use symbolic constants in s390_prologue_frame_unwind_cache

In s390_prologue_frame_unwind_cache there are two loops using a hardcoded
constant 16:
...
  for (i = 0; i < 16; i++)
    if (s390_register_call_saved (gdbarch, S390_R0_REGNUM + i)
  ...
  for (i = 0; i < 16; i++)
    if (s390_register_call_saved (gdbarch, S390_F0_REGNUM + i)
...

Fix this by using symbolic constants S390_NUM_GPRS and S390_NUM_FPRS instead.

Tested on s390x-linux, by rebuilding.

Approved-By: Andreas Arnez <arnez@linux.ibm.com>