Reimplement support for "maint print registers" with no running inferior yet

Reimplement support for "maint print registers" with no running inferior yet

A following patch will change the default target_thread_architecture
method, like this:

   struct gdbarch *
   default_thread_architecture (struct target_ops *ops, ptid_t ptid)
   {
  -  return target_gdbarch ();
  +  inferior *inf = find_inferior_ptid (ptid);
  +  gdb_assert (inf != NULL);
  +  return inf->gdbarch;
   }

This is because target_gdbarch is really just
current_inferior()->gdbarch, and it's wrong to return that
architecture when the inferior of the passed in PTID is NOT the
current inferior -- the inferior for PTID may be running a different
architecture.  E.g., a mix of 64-bit and 32-bit inferiors in the same
debug session.

Doing that change above however exposes a problem in "maint print
registers", caught be the testsuite:

 -PASS: gdb.base/maint.exp: maint print registers
 +FAIL: gdb.base/maint.exp: maint print registers (GDB internal error)
...
  gdb/inferior.c:309: internal-error: inferior* find_inferior_pid(int): Assertion `pid != 0' failed.
  A problem internal to GDB has been detected,

The call stack looks like this:

  #0  0x000000000068b707 in internal_error(char const*, int, char const*, ...) (file=0xa9b958 "gdb/inferior.c", line=309, fmt=0xa9b8e0 "%s: Assertion `%s' failed.") at gdb/common/errors.c:54
  #1  0x00000000006e1c40 in find_inferior_pid(int) (pid=0) at gdb/inferior.c:309
  #2  0x00000000006e1c8d in find_inferior_ptid(ptid_t) (ptid=...) at gdb/inferior.c:323
  #3  0x00000000007c18dc in default_thread_architecture(target_ops*, ptid_t) (ops=0xf86d60 <dummy_target>, ptid=...)
      at gdb/target.c:3134
  #4  0x00000000007b5414 in delegate_thread_architecture(target_ops*, ptid_t) (self=0xf86d60 <dummy_target>, arg1=...)
      at gdb/target-delegates.c:2527
  #5  0x00000000007647b3 in get_thread_regcache(ptid_t) (ptid=...) at gdb/regcache.c:466
  #6  0x00000000007647ff in get_current_regcache() () at gdb/regcache.c:475
  #7  0x0000000000767495 in regcache_print(char const*, regcache_dump_what) (args=0x0, what_to_dump=regcache_dump_none)
      at gdb/regcache.c:1599
  #8  0x0000000000767550 in maintenance_print_registers(char const*, int) (args=0x0, from_tty=1)
      at gdb/regcache.c:1613

I.e., the test does "maint print registers" while the inferior is not
running yet.  This is expected to work, and there's already a hack in
get_thread_arch_regcache to make it work.

Instead of pilling on hacks in the internal of regcache and
target_ops, this commit moves the null_ptid special casing to where it
belongs -- higher up in the call chain in the implementation of "maint
print registers" & co directly.

gdb/ChangeLog:
2017-10-04  Pedro Alves  <palves@redhat.com>

	* regcache.c (get_thread_arch_regcache): Remove null_ptid special
	case.
	(regcache_print): Handle !target_has_registers here instead.